JPS6319979B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to materials for gas diffusion electrodes suitable for use as, for example, fuel electrodes and oxidizer electrodes of hydrogen-oxygen fuel cells, air electrodes and oxidizer electrodes of air-zinc batteries, constituent electrodes of galvanic gas sensors, and the like. Conventionally, the following various types of gas diffusion electrodes have been used or proposed, but each has advantages and disadvantages. Porous graphite plate This has good electronic conductivity and air permeability. However, its mechanical strength is weak and it is difficult to use it in a thin-walled form, which limits the ability to reduce the weight and size of the entire battery. A sheet-like material made by binding particulate conductive substances such as activated carbon, graphite particles, and carbon fibers with fluororesin (e.g., Japanese Patent Application Laid-Open No. 1983-97133).This is a relatively thin wall and has comparative mechanical strength. You can get a good one. However, it has a drawback in that it lacks breathability. Therefore, in order to improve the air permeability of the sheet-like material, an attempt was made to manufacture the sheet-like material by blending an appropriate extractable pore-forming agent, and then to make the sheet porous by removing the blended pore-forming agent. However, such porous treatment reduces the electronic conductivity and mechanical strength of the sheet. Sheet materials made of intertwined fibers, such as water-repellent carbon fiber non-woven fabrics and woven fabrics, and mixed papers of carbon fibers and polytetrafluoroethylene fibers.These have considerably excellent properties in terms of electronic conductivity, air permeability, and mechanical strength. However, the fibers themselves are quite thick, and the porous flesh is composed of coarse intertwining of these fibers, so the size of the voids or pores formed in each part of the flesh is uneven, which makes the water repellent is not held uniformly on each part of the sheet surface. Also, overall water permeability is low. In view of the above, the present invention provides good electronic conductivity, gas permeability, and sufficient uniform water repellency in each part necessary for gas diffusion electrodes, and also maintains these performances stably over a long period of time ( Long lifespan), light specific gravity, thin walls, and sufficiently excellent mechanical strength can be obtained.It is not limited to sheets, but also tubes, cylinders, rods, etc., large plates or large sizes. It can be easily mass-produced with good yield. The purpose of the present invention is to provide a novel material for gas diffusion electrodes having the following characteristics. The gist of this is that the material has a large number of micronodules (nodes) 1 of polytetrafluoroethylene resin containing conductive powder, and that the material comes out from each node 1 and connects the nodules with each other in a three-dimensional manner. An entirely continuous fine porous structure consisting of a large number of polytetrafluoroethylene resin fibrils 2 containing no conductive powder, and each of the fine nodules partially touching or being continuous with each other. Gas diffusion electrode material 3 has the following properties. That is, the present inventors have developed the above-mentioned special fleshy polytetrafluoroethylene resin (hereinafter abbreviated as PTFE).
As a result of manufacturing a continuous fine porous structure 3 and examining its characteristics, we found that: A. Each micro nodule 1 in the porous flesh contains conductive powder, and each of these nodules is partially attached to each other. Since they are in contact or continuous, the entire material 3 has good electronic conductivity. B The porous structure composed of the three-dimensional network combination of each micronodule 1 and each microfiber 2 has a high porosity and provides good gas permeability throughout the material 3. C Micronodules 1 The spaces between each other are filled with numerous fine fibers 2
is spread out in a three-dimensional network, and there is no unevenness in the size of voids or pores in each part of the material, and each of the fine fibers 2 does not substantially contain conductive powder, and each fine fiber maintains the strong water repellency inherent to PTFE. Therefore, liquid that attempts to penetrate into the porous flesh of the material 3 is blocked by the strong water repellency of each fine fiber 2, and the material has uniform water repellency and water permeability throughout the material. show. D The electronic conductivity of A, the air permeability of B, and the water repellency/water permeability resistance of C are maintained stably over a long period of time. E It has a light specific gravity. F It is also possible to easily provide a catalyst function as described below. G For example, by using the manufacturing method described below, it is possible to obtain products that are thin and have sufficient mechanical strength, and are not limited to sheet-like products, but also products with shapes such as tubes, cylinders, and rods, large plates, or large-sized products. It can also be easily mass-produced with good yield. The present invention was completed by discovering that this material fully satisfies almost all the conditions required as a material for gas diffusion electrodes. The above-mentioned special material 3 of the present invention is, for example,
Publication No. 14178, Publication No. 44664, Publication No. 14178, Publication No. 44664, Publication No. 14178, Publication No. 44664, Publication No. 14178.
By applying the method for producing a PTFE porous structure disclosed in Japanese Patent No. 18991 and the like, it is possible to easily mass-produce the porous PTFE structure with a high yield through the following process steps (1) to (4). (1) Prepare a paste-like mixture whose basic ingredients are PTFE fine powder, conductive substance powder, and liquid lubricant. (2) The mixture is formed into a sheet, tube, rod, etc. by compression, extrusion, rolling, or a combination thereof. (3) After removing the liquid lubricant from the molded product by means such as heating and extraction, the molded product is stretched in at least one direction according to the procedure described in the above publication. (4) The above stretched product (unfired product) may be used as a final product, but if necessary, the stretched product may be further rolled or compressed with a roll or press plate, or heat treated (completely baked). or incomplete firing) H, or process P and then process H, or process H first and then process P, or process P, then process H, and then process P again. Final product. In preparing the paste-like mixture as the raw material in (1) above, the conductive substance powder is specifically carbon black powder alone, or carbon black powder and various other conductive powders, such as graphite powder. Activated carbon powder, carbon fiber, metal powder (platinum,
Mixed powders with gold, tantalum, titanium, nickel, etc.), metal oxide powders, Raney metal powders, etc. can be used. Further, it is preferable that the conductive material powder has a particle size of at least 1 μm or less. particle size
If a material with a diameter of 1 μm or more is blended, it becomes extremely difficult to stretch the material in step (3), making it difficult to adjust the pore size of the material and making it difficult to obtain a good target material. In addition, the amount of conductive material powder blended is such that the volume resistivity of the material (PTFE fine powder + conductive material powder) before the stretching process in step (3) is 1.0Ω-
Blend the relevant quantities so that it is less than cm. That is, even if a material whose volume resistivity before stretching is 1.0 Ω-cm or more is stretched, the conductivity of the material after stretching is low, making it difficult to use as an electrode. The conductive material powder to be blended contains at least carbon black powder in an amount of 7 to 80% by weight, preferably 15 to 70% by weight of the total material weight (PTFE fine powder + conductive material powder). According to experiments, if carbon black powder is not included, no matter how high the conductivity of the material before stretching is due to the presence of conductive substance powder other than carbon black, the conductivity after stretching is significantly lower. As a result, the electron conductivity necessary for an electrode cannot be obtained. Liquid lubricants include petroleum, solvent naphtha,
Various lubricants such as liquid hydrocarbons such as white oil as exemplified in the above-mentioned publication can be used, and the blending amount is generally set in the range of about 20 to 200% by weight. The whole paste-like mixture of PTFE fine powder, conductive substance powder, and liquid lubricant can be prepared by, for example, (a) a predetermined amount of conductive substance powder for the PTFE dispersion, or a mixture in which the conductive substance powder has been previously dispersed in water. PTFE
The conductive substance powder is aggregated on the particles (if the aggregation does not occur due to the above stirring, a flocculant such as Freon is mixed in). Then, a predetermined amount of liquid lubricant is added to the agglomerated mixture and mixed well. (b) The PTFE fine powder and the conductive material powder are mixed uniformly using a rotary mixer, and then a liquid lubricant is added to the mixture and mixed well. C. A mixture of conductive material powder and liquid lubricant
Add to the PTFE fine powder that was previously placed in a blender and mix well. It can be well prepared by means such as. If desired, suitable amounts of auxiliary raw materials such as water repellency enhancing powder such as wax graphite powder, reinforcing material powder such as fluoro rubber, and coloring pigments may be blended. Further, by blending a catalytic substance, material A having a catalytic function can be obtained, which will be described later. The paste-like mixture prepared above is molded into a desired shape in the molding step (2), and then the liquid lubricant is removed from the molded product and then subjected to the stretching treatment in (3).
The flesh of the molded material changes into a continuous microporous structure consisting of numerous PTFE micro nodules 1 and PTFE fine fibers 2 that come out from each of the nodules and connect the nodules three-dimensionally. . The physical properties of the porous structure can be determined by setting various manufacturing conditions such as the stretching direction, stretching ratio (generally about 1.2 to 1.5 times the original length of the material), stretching temperature, and stretching ratio per unit time. It can be adjusted as desired within a wide range. For example, porosity (porosity) 50-95%, maximum pore diameter
0.1~20μm, density 0.2~1g/ ^cm3 , Gurley number 1 second or more, ethanol valve point 0.2~
3Kg/cm ² , matrix tensile strength 514Kg/cm or more, wall thickness 1mm or more optional. In addition, Gurley number (Gurley number) is the diameter
A 2.54 cm material cross section was heated to 100 c.c. under a pressure of 12.7 cm H ₂ O.
represents the time required for air to pass through. In this case, the present inventors discovered a peculiar phenomenon in which most of the conductive substance powder mixed into the material in the PTFE porous structure obtained by the stretching process was contained in the individual nodules 1. , a phenomenon of nodule concentration occurs in which almost no individual fine fibers 2 are contained, and each nodule 1 containing a concentrated amount of conductive substance powder is partially in contact with or continuous with each other. When the porous structure is PTFE, the porous structure shows good electronic conductivity as a whole, and the individual fine fibers 2 do not substantially contain conductive substance powder and maintain the strong water repellency inherent to PTFE. It has been found that since the porous structure exists across the spaces between the nodules 1, the porous structure exhibits sufficient water repellency uniformly throughout the entire portion. In the above, a porous structure in which the nodes 1 are partially in contact with each other or are continuous, is produced by: 1) The stretching treatment of the material described in (3) above is performed so that each node 1 is not completely separated from each other. It is carried out at a relatively small range of stretching ratio, stretching speed, etc. that does not cause the stretching. 2. Once stretched at an arbitrary stretching ratio, stretching speed, etc., the porous structure is rolled or compressed to make the individual nodules 1 partially in contact with each other or continuous. It can be easily configured by means such as Regardless of which method 1) or 2) above is taken, the porous body finally obtained has a maximum pore diameter of 10 μm or less, preferably 5 μm or less, and an air permeability of 5 in Gurley number. The stretching conditions such as the stretching ratio and the stretching speed are set so that the stretching time is within the range of ~500 seconds, or the pressing force against the porous body during rolling or compression treatment is set. If the cap is closed and the maximum pore diameter is 10 μm or more and the Gurley number is 5 seconds or less, the permeation pressure will decrease, allowing electrolyte, generated water, etc. to easily enter, and the mechanical strength will also decrease significantly. Furthermore, if the Gurley number is 500 seconds or more, the air permeability will be insufficient and the material will not be able to fully function as a gas diffusion electrode. The unfired PTFE porous material obtained up to the stretching process in (3) above or by further rolling or compression treatment may be used as it is as a gas diffusion electrode material, but it may be further used in the above-mentioned (4). ), it may be used in the form of complete firing (heating and firing at a temperature of 327°C or higher, the melting point of PTFE) or incomplete firing (heating at a temperature of 327°C or lower). An unfired product has a porous structure with a more uniform pore size than a completely fired product. A completely fired product has improved mechanical strength and electronic conductivity than an unfired product (this improvement in properties is remarkable when firing is performed in the range of 327 to 370°C). The incompletely fired product has intermediate characteristics between the unfired product and the completely fired product. The above-mentioned material rolling or compression treatment may be carried out after the above-mentioned heat treatment (complete or incomplete firing) of the material, or may be carried out twice in total, before and after the heat treatment. Alternatively, the rolling or compression treatment and/or heat treatment of the material may be performed simultaneously with the pressing and/or heat treatment step for adhering the current collector and/or catalyst layer to the material. In addition, for the purpose of preventing the conductive material powder contained in the porous body and the catalyst from flowing out when the catalyst is contained in the porous body as described below, a dispersion of PTFE or FEP, for example, may be used as a filler. It is also effective to fix the conductive powder or catalyst to the porous body by impregnating the porous body with a suitable amount of silicone or the like and firing the porous body. Alternatively, the stretching treatment of the material may be carried out after forming the paste-like mixture as the raw material, with the liquid lubricant still contained in the formed product, and then removed. However, better results can be obtained if the liquid lubricant is removed in advance and then the stretching is performed. Next, if it is desired that the porous body also has a catalytic function, any of the following methods may be employed. a. As the conductive material powder, use is made of powder on which a catalytic material such as platinum black is supported in advance. b. As PTFE powder, a catalyst material is supported by applying a catalyst precursor to the powder and depositing it as a catalyst material by chemical or/and physical means such as heating, hydrolysis, reduction, etc. use c Example of method for preparing the paste-like mixture as the raw material mentioned above
In (a), PTFE dispersion is mixed with a catalyst material in addition to conductive material powder.
A conductive material powder containing a catalyst material is aggregated into powder. d In the same preparation method example (b), a catalyst material is added to the PTFE powder in addition to the conductive material powder, and the three are uniformly mixed. e In the same preparation method (c), a mixture of conductive material powder, catalyst, and liquid lubricant is added to the PTFE powder previously charged in a V-shaped blender and mixed well. f Once the porous body is produced, it is impregnated with a solvent in which a catalyst substance is dispersed, and then dried. g. A precursor of a catalyst substance is contained in the porous body once produced, and the precursor is precipitated as a catalyst substance by chemical and/or physical means such as heating and hydrolytic reduction. h. An air permeable film or sheet-like material containing a catalyst substance is laminated on the surface of the porous body by pressure bonding, heat fusion, etc. etc. The porous material according to the present invention described above is used as a gas diffusion electrode either alone or in a form in which a current collector, a water-repellent porous membrane, etc. are integrated therewith. The porous body has almost all the characteristics required as a gas diffusion electrode as described in the above items A to G, such as hydrogen-oxygen fuel cells, air-zinc batteries, and galvanic gas sensors. It is extremely effective and suitable as a material for constituent electrodes. Examples will be described below. Example 1 Mixtures of the following four types of PTFE powder and conductive substance powder (1) to (4) with different blending ratios were prepared by agglomeration method (Special Publication No. 52
-34653 Publication). (1) PTFE powder...80% (wt%, the same below) Conductive carbon black...20% (manufactured by Agzo Chemie, Netherlands, trade name: Ketschen Black EC, the same below) (2) PTFE powder... ……55% Conductive carbon black……20% Graphite powder with average particle size of 0.3μm……25% (3) PTFE powder……50% Conductive carbon black……20% Average particle size 0.2 μm graphite fluoride powder...30% (4) PTFE powder...40% Conductive carbon black...60% Each of the above mixtures is mixed with a liquid lubricant (solvent naphtha) to form a paste. A mixture was prepared, and the mixture was compressed and ram extruded from a fish tail to produce sheet shaped moldings each having a thickness of 1 mm. Next, each sheet-shaped molded product was further rolled in a direction perpendicular to the extrusion direction to form a thin sheet with a thickness of 0.3 mm. And for each thin-walled molded sheet,
After preheating to 300℃ and stretching 1.7 times in the uniaxial direction, the length of the stretched sheet in the stretching direction is fixed to prevent heat shrinkage, and then heating to 355℃ for complete firing. A continuous microporous PTFE film containing conductive material powder was obtained. Table 1 shows the volume resistivity (thickness direction resistance) after roll rolling and before stretching, the same resistance after stretching and before firing, and the same resistance after firing for the four types of films. Indicates air permeability (Gurley number).

[Table] Comparative Example 1 (When carbon black is not blended) Using a mixture of 70% PTFE powder and 30% graphite powder with an average particle size of 0.3 μm as a conductive substance powder as a raw material, this was used as Example 1. The volume resistivity values before stretching, after stretching, and after firing of a porous film made using the same procedure (but stretching 1.5 times) are shown in the lower column of Table 1. In other words, if carbon black is not added, even if the material has high electrical conductivity (that is, low volume resistivity) before stretching, its electrical conductivity will drop significantly after stretching, making it unsuitable for use as an electrode. do not have. Comparative Example 2 (When the particle size of the conductive substance powder is large) Same as Example 1 using a mixture of PTFE powder...50%, carbon black...20%, and graphite powder with an average particle size of 3 μm...30% as raw materials. The sheet was processed into a 0.3 mm thick sheet using the procedure described above, and then stretched 1.7 times, but the sheet broke and it was not possible to form a porous film. Example 2 (Example of use as an electrode) A mixture of 80% PTFE powder + 20% conductive carbon black is made by the coagulate method, and a liquid lubricant (approximately 50%) is mixed into the mixture to form a paste mixture. was prepared. The paste-like mixture was compressed and extruded by ram extrusion into a rod-like product with an outer diameter of 25 mm, which was further rolled to form a ribbon-like sheet with a thickness of 0.3 mm and a width of 100 mm. The sheet was then placed in an atmosphere at 150°C to vaporize and completely remove the liquid lubricant. The volume resistivity value of the material at this stage was 0.8 Ω-cm. (1) This unstretched sheet itself. (2) The unstretched sheet was then stretched 4 times in the longitudinal direction in an atmosphere at 300°C to make the flesh porous (thickness: 0.28 mm), and the sheet was further rolled with a roll to a thickness of 0.16 mm. (unfired compressed porous sheet). (3) The sample treated as in sample 2 above was further fixed at both longitudinal ends to prevent heat shrinkage, and then heated to 310 mm.
Heat treated in an atmosphere at ℃ for 5 minutes (incompletely fired compressed porous sheet). (4) The sample treated as in sample 2 above was further fixed at both longitudinal ends to prevent heat shrinkage and heated to 360°C.
(Completely fired compressed porous sheet) heat treated for 5 minutes in an atmosphere of The above four types of sheets were made, and one side of each sheet was coated with a platinum black catalyst containing a small amount of FEP powder at a catalyst weight of 4 mg/cm ² , and then kept in an atmosphere at 290°C for 5 minutes. and bound the catalyst. Each of the sheets (1) to (4) subjected to the above catalyst binding treatment was used as an electrode material for a hydrogen electrode and an oxygen electrode, respectively.
Ni mesh/electrode/electrode catalyst surface/electrolyte (KOH35% by weight solution)/electrode catalyst surface/electrode/Ni
We created a hydrogen-oxygen fuel cell with a mesh/hydrogen sequential configuration, and measured the current density when the output reached 0.7V. The results are shown in Table-2. Table 2 Electrode material Current density mA/ ^cm 2Example 2-(1) 30 〃-(2) 62 〃-(3) 71 〃-(4) 75 As is clear from the results of Table-2, the electrode material according to the present invention It was found that the sheets (2) to (4) having a stretched porous structure can have a higher current density than the unstretched sheet (1) in which the conductive material powder is simply dispersed in the PTFE resin. Furthermore, when the battery characteristics were measured over time, no decrease in current was observed even after 1000 hours. In addition, the sheet strength of stretched sheets (2) to (4) is better than that of unstretched sheet (1), and among the three sheets, the strength is higher in the order of (2), (3), and (4). Excellent strength. In addition, the electronic conductivity of the sheet is 4 after compression treatment than before compression treatment in both the thickness direction and surface direction of the sheet.
Improved twice. Example 3 The fully fired compressed porous sheet (4) produced in Example 2 was immersed in a 10% by weight isopropyl alcohol solution of chloroplatinic acid for 24 hours, then taken out and fixed at both ends in the stretching direction. , heat-dried under reduced pressure, and then heated to 210°C. The porous structure sheet according to the present invention has a platinum catalyst supported thereon by repeating this solution immersion → heating operation once again, reducing it at 210°C for 4 hours in a hydrogen stream, and cooling it to room temperature in nitrogen gas. I got it. The amount of platinum supported on this sheet was 1 mg/cm ² . Carbon paper was bonded to one side of this porous structure sheet via FEP and used as an electrode material to form an oxygen/Ni mesh/electrode/electrolyte (KOH 35% by weight) impregnated matrix/electrode/Ni.
An alkaline electrolyte matrix type hydrogen-oxygen fuel cell with a mesh/hydrogen ordered structure was constructed. The results of measuring the output characteristics of this battery are shown in the graph of FIG. Example 4 The fully fired compressed porous sheet A of (4) produced in Example 2 and the platinum catalyst-supported porous sheet B produced in Example 3 were stacked together with FEP powder sandwiched between them at 300°C.
Both sheets were bonded together by passing them between heated rolls. Next, on the sheet A side of the adhesive sheet
Carbon paper was bonded via FEP and used as an electrode material. Using this material, a solid electrolyte fuel cell with a sequential configuration of air/stainless steel net/electrode material/ion exchange membrane/electrode material/stainless steel net/hydrogen was fabricated. For both the air electrode and the hydrogen electrode, the electrode material was used with the carbon paper side facing the stainless steel net side. Nafion #120 was used as the solid electrolyte. The output characteristics of this battery are shown in the graph of Figure 3. or
When continuously operated at 50 mA/cm ² and 0.6 V, almost no change was observed even after 1000 hours. Example 5 PTFE powder...35% Conductive carbon black...65% on which 10% of platinum (catalyst) was supported by a conventional method was mixed by the coagulate method, and a liquid lubricant (approximately 100%) was mixed therein. , compressed and ram extruded to form a sheet with a thickness of 1 mm. Next, this sheet was rolled to a thickness of 0.3 mm using rolls. This is called sheet A. Separately, a mixture of 40% PTFE powder, 25% conductive carbon black, and 35% graphite fluoride was used to form a sheet 0.3 thick in the same manner as sheet A above.
A sheet of mm was formed. This is called sheet B. The above sheets A and B were stacked and rolled again to form a two-layer structure sheet C having a total thickness of about 0.3 mm. After completely removing the liquid lubricant by heating this sheet C to 220°C in a nitrogen stream,
It was stretched twice in an atmosphere at ℃. After applying FEP dispersion (manufactured by Mitsui Fluorochemicals, Teflon #120) to the sheet B side (water-repellent and conductive sheet surface containing graphite fluoride) of this stretched sheet, carbon paper was applied. Approx. 0.2Kg/ ^cm2
The sheet was fired by heating it to 350°C while being pressed with a weak force, and at the same time, carbon paper was laminated to create a sheet material for electrodes. The amount of platinum supported on this material was 0.5 mg/cm ² . Using this material as an electrode material, hydrogen/SUS mesh/electrode (carbon paper/water-repellent conductive layer/catalyst layer)/KOH (35% by weight) electrolyte/electrode (catalyst layer/water-repellent conductive layer/carbon paper)/ We created an alkaline electrolyte fuel cell with a SUS mesh/oxygen sequence. The output characteristics of this battery are shown in the graph of FIG. Moreover, no change in this property was observed even after 2000 hours.

[Brief explanation of the drawing]

FIG. 1 is a model diagram of the texture of the material of the present invention, and FIGS. 2 to 4 are graphs of output characteristics of batteries constructed in Examples. 1 is a nodule, 2 is a fine fiber, and 3 is the whole porous flesh composed of the nodule and the fine fiber.

Claims (1)

[Scope of Claims] 1. A material whose flesh consists of numerous micronodules of polytetrafluoroethylene resin containing conductive substance powder, and a conductive substance that comes out from each of these nodules and connects the nodules three-dimensionally. It is characterized by being a continuous fine porous structure consisting of a large number of polytetrafluoroethylene resin fine fibers containing no powder, and in which each fine nodule is partially in contact with or continuous with each other. Materials for gas diffusion electrodes. 2. The gas diffusion electrode according to claim 1, wherein the conductive substance powder is carbon black powder, or a mixture of carbon black powder and one or more other conductive powders having a particle size of 1 μm or less. Materials for use. 3. The gas diffusion electrode material according to claim 1, which contains at least 7 to 80% by weight, preferably 15 to 70% by weight of carbon black powder as the conductive substance powder. 4 A mixture of polytetrafluoroethylene powder and conductive substance powder whose overall volume resistivity is 1.0.
The material for a gas diffusion electrode according to claim 1, which is produced by stretching a material having a diameter of Ω-cm or less to make the flesh continuous and microporous. 5. The gas according to claim 1, wherein the maximum pore diameter of the continuous microporous flesh is 10 μm or less, preferably 5 μm or less, and the air permeability is in the range of 5 to 500 seconds in terms of Gurley number. Materials for diffusion electrodes. 6. The gas diffusion electrode material according to claim 1, which contains a catalyst substance.