Gas diffusion electrode substrate products and polymer electrolyte fuel cells

Abstract

The present invention addresses the problem of reducing the aging time of a fuel cell.　In order to solve the above-described problem, a gas diffusion electrode base material product according to the present invention has a sulfuric acid content of 1.1 μg / cm2 or less. In addition, this gas diffusion electrode base material product is incorporated in a polymer electrolyte fuel cell according to the present invention.

Description

[Technical Field]

[0001] The present invention relates to a gas diffusion electrode substrate product suitable for use in electrodes of polymer electrolyte fuel cells. [Background technology]

[0002] The electrodes of a polymer electrolyte fuel cell (hereinafter simply referred to as "fuel cell") generally consist of a catalyst layer formed in contact with an electrolyte membrane and a gas diffusion electrode substrate laminated on the surface of the catalyst layer. A fuel cell has a structure in which this is sandwiched between separators.

[0003] After assembling the various components described above to form a fuel cell, the cell undergoes a process called aging, which aims to activate the catalyst and verify the absence of defects before it is ready for actual use. Since aging requires a specific combination of power generation conditions to generate electricity for a certain period of time, shortening the aging time is a key challenge for improving fuel cell production efficiency. For example, Patent Document 1 discloses a technology for shortening the aging time by periodically changing the flow rate of an inert gas mixed with an oxidant gas during aging. Patent Document 2 also discloses a technology for shortening the aging time by coating the electrolyte membrane layer, which is the main component, with a different polymer layer to improve contact with the electrode. Furthermore, Patent Document 3 discloses a technology for shortening the aging time by efficiently forming proton conduction paths by impregnating porous carbon, which serves as a catalyst support, with an acid. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-128976 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-295572 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-238485 [Patent Document 4] International Publication No. 2015 / 125750 Summary of the Invention [Problem to be solved by the invention]

[0005] Another purpose of aging fuel cells is to remove impurities from the catalyst surface and to dissolve acids contained in the various components in the cell using water produced during power generation. Therefore, the acid concentration and pH of the discharged water are sometimes used as indicators to determine when aging should be completed. In other words, if the various components in the cell contain large amounts of acid-producing components, the time required for aging increases, reducing the production efficiency of fuel cells. The present invention aims to shorten the aging time for fuel cells. [Means for solving the problem]

[0006] As a result of investigating the origin of the acid generated during aging, the present inventors discovered that a certain proportion of the acid is sulfuric acid eluted from a gas diffusion electrode having a microporous layer containing carbon black, in which carbon paper is immersed in a water-repellent resin as in Patent Document 4. Based on this finding, the present inventors have made the present invention to solve the above-mentioned problems, which is characterized by the fact that the sulfuric acid content is 0.5 μg / cm 2 The present invention relates to the following gas diffusion electrode substrate product and a solid polymer fuel cell incorporating the same. [Effects of the Invention]

[0007] By using the gas diffusion electrode substrate product of the present invention, the aging time can be shortened. DETAILED DESCRIPTION OF THE INVENTION

[0008] <Gas diffusion electrode substrate products> In this specification, the term "gas diffusion electrode substrate product" refers to a new gas diffusion electrode substrate after manufacture, and excludes a gas diffusion electrode after it has been incorporated into a fuel cell and started generating electricity. Typically, the term "gas diffusion electrode substrate product" in this specification refers to a gas diffusion electrode substrate that has been wound into a roll after manufacture, or a new gas diffusion electrode substrate that has been cut from the rolled gas diffusion electrode substrate and is in a state before it is incorporated into a fuel cell. However, in this specification, the term "electrode substrate" may hereinafter be used to refer to a "gas diffusion electrode substrate product."

[0009] In a first preferred embodiment of the gas diffusion electrode substrate product of the present invention, the gas diffusion electrode substrate product preferably consists essentially of a conductive porous body. The conductive porous body is typically a porous body having a pore structure with an average pore diameter of 10 μm or more as measured by mercury intrusion porosimetry. The upper limit of the average pore diameter is not particularly limited, but is usually about 100 μm. As such a conductive porous body, a conductive porous body containing carbon fiber, such as a carbon fiber fabric, a carbon fiber paper, a carbon felt, or a carbon paper, is preferably used, and a conductive porous body made of carbon fiber is more preferably used. The conductive porous body preferably has spring-like properties (springiness) to absorb thickness changes in the electrolyte membrane during power generation and to provide good fastening force against compression when stacking and assembling each electrode component into a cell. From this perspective, the conductive porous body is preferably a porous body made of carbon fiber bound with a resin carbide, and particularly preferably a porous body made of a carbon fiber paper bound with a resin carbide, i.e., carbon paper or carbon felt.

[0010] The conductive porous body serves to diffuse gases such as oxygen and hydrogen, which are fuels for fuel cells, as well as the water (water vapor) produced. Therefore, the thickness of the conductive porous body is preferably 220 μm or less. To further improve gas diffusibility, the thickness of the conductive porous body is preferably 150 μm or less, and more preferably 100 μm or less. On the other hand, the thinner the conductive porous body, the better the gas diffusibility, but if it is too thin, handling becomes difficult, so the practical lower limit is 70 μm.

[0011] Examples of carbon fibers used in conductive porous bodies include polyacrylonitrile (PAN)-based, pitch-based, and rayon-based carbon fibers. Among these, PAN-based carbon fibers are preferred due to their excellent mechanical strength and processability. The carbon fibers constituting the carbon paper preferably have an average single fiber length (hereinafter referred to as "carbon fiber length") within the range of 3 to 20 mm, more preferably 5 to 15 mm. A carbon fiber length of 3 mm or more, more preferably 5 mm or more, tends to result in a carbon fiber sheet with excellent mechanical strength, electrical conductivity, and thermal conductivity. On the other hand, a carbon fiber length of 20 mm or less, more preferably 15 mm or less, tends to provide excellent dispersion of carbon fibers during the production of a carbon fiber paper, making it easier to obtain a homogeneous carbon fiber sheet. Carbon fibers having such a carbon fiber length can be obtained by, for example, cutting continuous carbon fibers to the desired length. Carbon felt substrates can be obtained by cutting carbon fiber precursor fibers to lengths of approximately several tens of millimeters (generally 40 mm to 100 mm), processing them into a web, and then entangling the fibers by needle punching or the like to form a nonwoven fabric substrate, followed by carbonization.

[0012] As the resin used to produce the resin char that binds the carbon fibers, thermosetting resins such as phenolic resin, epoxy resin, melamine resin, and furan resin are particularly preferred. Furthermore, to obtain even higher electrical and thermal conductivity, carbon particles may be contained in the resin char. Examples of carbon particles that can be contained in the resin char include graphite such as flake graphite, flaky graphite, amorphous graphite, artificial graphite, expanded graphite, and flake graphite, carbon nanotubes, carbon nanofibers, and milled carbon fiber.

[0013] The conductive porous body in the present invention is preferably treated with a water-repellent resin to make it water-repellent so that water generated during power generation in the fuel cell can be quickly discharged outside the system. That is, the conductive porous body preferably contains a water-repellent resin, and when a porous body made by binding carbon fibers with a resin carbide is used as the conductive porous body, it is preferable that the water-repellent resin is attached to the carbon fibers. In this specification, when the conductive porous body contains a water-repellent resin, the water-repellent resin is also referred to as the "conductive porous body."

[0014] Fluororesins are preferably used as such water-repellent resins. Examples of fluororesins include PTFE (polytetrafluoroethylene), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PFA (perfluoroalkoxy fluoride resin), ETFA (ethylene-tetrafluoroethylene copolymer), PVDF (polyvinylidene fluoride), and PVF (polyvinyl fluoride). The fluororesin content of the conductive porous body is preferably 0.1% by weight to 20% by weight, based on 100% by weight of the conductive porous body without the fluororesin. If the content is less than 0.1% by weight, the water repellency may be insufficient, and if it exceeds 20% by weight, the electrical resistance may deteriorate. Here, fluororesins may contain sulfur. In this invention, however, a fluororesin with a sulfur content of 50 ppm or less is preferably used as the water-repellent resin, and more preferably a fluororesin with a sulfur content of 30 ppm or less is more preferably used. That is, a first preferred embodiment of the gas diffusion electrode substrate product of the present invention includes a conductive porous body made of carbon fibers, and the carbon fibers are adhered with a fluororesin having a sulfur content of preferably 50 ppm or less, more preferably 30 ppm or less.

[0015] A second preferred embodiment of the gas diffusion electrode substrate product of the present invention preferably comprises a conductive porous body made of carbon fiber and a microporous layer containing carbon powder provided on at least one surface of the conductive porous body. The conductive porous body is the same as the conductive porous body described above in the first preferred embodiment of the gas diffusion electrode substrate product of the present invention. The microporous layer is typically a porous layer having an average pore diameter of 0.01 μm to 1 μm as measured by mercury intrusion porosimetry.

[0016] The microporous layer contains carbon powder. By including carbon powder in the microporous layer, fine pores can be formed and electrical conductivity can be imparted. Examples of carbon powder include carbon black, graphite, expanded graphite, flake graphite, carbon nanotubes, and carbon nanofibers. Among these, carbon black is preferred from the viewpoints of cost and ease of handling. While carbon powders such as carbon black may contain sulfur, in the present invention, it is preferable that the carbon powder have a low sulfur content. Specifically, in a second preferred embodiment of the gas diffusion electrode substrate product of the present invention, the sulfur content of the carbon powder is preferably 3000 ppm or less, and more preferably 2500 ppm or less. The lower limit of the sulfur content is not particularly limited, but is typically about 1 ppm.

[0017] A second preferred embodiment of the gas diffusion electrode substrate product of the present invention preferably contains carbon black that has been heat-treated in an inert atmosphere at 2000°C or higher for 10 minutes or longer. By performing such heat treatment, even when carbon black with a sulfur content of more than 3000 ppm is used, the sulfur content of the carbon black can be reduced to 3000 ppm or less before being incorporated into the microporous layer. The upper limit of the heat treatment temperature is not particularly limited, but is usually around 3000°C.

[0018] The microporous layer preferably has water repellency, similar to the conductive porous body described above. Therefore, the microporous layer preferably contains a water-repellent resin in addition to carbon powder. As the water-repellent resin contained in the microporous layer, a fluororesin similar to that used in the conductive porous body described above is preferably used, and a resin with a low sulfur content is also preferably used, so a detailed description thereof will be omitted here.

[0019] A second preferred embodiment of the gas diffusion electrode substrate product of the present invention is a conductive porous body made of carbon fibers, in which a fluororesin having a sulfur content of preferably 50 ppm or less, more preferably 30 ppm or less, is adhered to the carbon fibers.

[0020] The gas diffusion electrode substrate product of the present invention has a sulfuric acid content of 1.1 μg / cm 2 The sulfuric acid content of the gas diffusion electrode substrate product is preferably 0.5 μg / cm 2 or less, more preferably 0.2 μg / cm 2 The sulfuric acid content is 1.1 μg / cm 2 If the sulfuric acid content exceeds 1.1 μg / cm 3 , the time required for aging will be longer. As a method for adjusting the sulfuric acid content to the above range, for example, the sulfuric acid content of the above components constituting the gas diffusion electrode substrate product is adjusted to 1.1 μg / cm 3 as a whole. 2 The following methods are available for adjusting the sulfuric acid content. The lower the sulfuric acid content, the better. There is no particular lower limit for the sulfuric acid content, but it is usually 0.01 μg / cm 2 That's about it.

[0021] The polymer electrolyte fuel cell of the present invention incorporates the gas diffusion electrode substrate product of the present invention. In addition to the gas diffusion electrode substrate product, the polymer electrolyte fuel cell may also incorporate a solid polymer electrolyte membrane, a catalyst layer, a separator, and the like.

[0022] <Method of manufacturing gas diffusion electrode substrate products> The gas diffusion electrode substrate product of the present invention can be produced, for example, by subjecting a conductive porous body to a water-repellent treatment, applying a microporous layer coating liquid to at least one surface of the body, and then sintering the body. The inventors' studies have revealed that when the various materials constituting the gas diffusion electrode substrate contain a certain amount or more of sulfur components (sulfur oxides or sulfuric acid), the sulfur components are oxidized during the sintering process, liberating sulfuric acid. In such cases, sintering at 400°C or higher and 500°C or lower can volatilize and remove the sulfur components. Sintering temperatures below 400°C may not adequately remove the sulfur components. Furthermore, sintering temperatures above 500°C may decompose the fluororesin binding the carbon powder in the microporous layer, resulting in excessive carbon powder and making it impossible for the microporous layer to maintain its layered structure. From this perspective, the sintering temperature is preferably 410°C or higher and 480°C or lower, and even more preferably 420°C or higher and 450°C or lower. Alternatively, sintering may be performed once at a general sintering temperature of 250°C or higher and lower than 400°C, and then further sintering may be performed at 400°C or higher and 500°C or lower. [Example]

[0023] Next, the gas diffusion electrode substrate product of the present invention will be specifically described using examples, but the present invention is not limited to these examples. The materials used in the examples, the methods for producing and evaluating the gas diffusion electrode substrate product, and the methods for evaluating it as a fuel cell are shown below.

[0024] [Sulfuric acid content in electrode substrate] Gas diffusion electrode substrate product approx. 9cm 2 The sample was cut and weighed, and the target components were extracted with 100 mL of ultrapure water. The extract was analyzed by ion chromatography (INTEGRION, Thermo Fisher Scientific) to quantify the amount of sulfuric acid in the electrode substrate. The quantitative value was divided by the area of ​​the gas diffusion electrode substrate product to determine the sulfuric acid content (μg / cm). 2 ) was sought.

[0025] [Sulfur content] The target substance was burned in an electric furnace at 1000°C, and the generated gas was absorbed in an absorbing solution. 100 μL of the absorbing solution was analyzed by ion chromatography (Dionex ICS1600) to quantify the sulfur content. The sulfur content (ppm) was calculated by dividing the measured value by the weight of the target substance used in the analysis.

[0026] [Aging test] A catalyst solution was prepared by sequentially adding 1.00 g of platinum-supported carbon (manufactured by Tanaka Kikinzoku Kogyo K.K., platinum loading: 50% by mass), 1.00 g of purified water, 8.00 g of Nafion (registered trademark) solution (5.0% by mass of Nafion (registered trademark) manufactured by Aldrich), and 18.00 g of isopropyl alcohol (manufactured by Nacalai Tesque, Inc.).

[0027] Next, the catalyst solution was sprayed onto "Naflon" (registered trademark) PTFE tape "TOMBO" (registered trademark) No. 9001 (manufactured by Nichias Corporation) cut to a size of 5 cm x 5 cm, and then dried at room temperature to obtain a platinum content of 0.3 mg / cm. 2 A PTFE sheet with a catalyst layer was produced. Next, a solid polymer electrolyte membrane "Nafion" (registered trademark) NRE-211CS (manufactured by DuPont) cut to 8 cm x 8 cm was sandwiched between two PTFE sheets with a catalyst layer, and pressed at 130°C for 5 minutes under a pressure of 5 MPa using a flat plate press, thereby transferring the catalyst layer to the solid polymer electrolyte membrane. After pressing, the PTFE sheets were peeled off, and a solid polymer electrolyte membrane with a catalyst layer was produced.

[0028] Next, the catalyst-coated solid polymer electrolyte membrane was sandwiched between two gas diffusion electrode substrates cut to 5 cm x 5 cm, and pressed at 130°C for 5 minutes under a pressure of 3 MPa using a flat press to produce a membrane electrode assembly. The resulting membrane electrode assembly was then sandwiched between separators and incorporated into a single fuel cell for evaluation. The separator used was a serpentine-type separator with a single channel, with a groove width, groove depth, and rib width all of 1.0 mm.

[0029] Using the fuel cell obtained in this way, unpressurized hydrogen was supplied to the anode side and unpressurized air to the cathode side to generate electricity. Both the hydrogen and air were humidified using a humidifying pot set at a temperature of 70°C. The humidity at this time was 100%. The utilization rates of oxygen in the hydrogen and air were set at 70 mol% and 40 mol%, respectively, and the cell temperature was set at 70°C. For aging, a current density of 1.2 A / cm was used. 2 Power generation at 0.4A / cm was maintained for 30 minutes, and then 2 and 1.2A / cm 2 The power generation was performed by alternating five times the current density of 1.2 A / cm2 for five minutes. 2 The water discharged from the cell after aging (the stage where power generation at room temperature was maintained for 30 minutes) and after aging was completed was collected and the pH of the water was measured.

[0030] [ refer to Example 1] Carbon black A, which had a sulfur content of 5,000 ppm, was heat-treated in an argon stream at 2,400°C for 10 minutes to remove the sulfur. After the heat treatment, the sulfur content of carbon black A (referred to as carbon black AH) was reduced to 50 ppm.

[0031] A microporous layer coating liquid was prepared by kneading 15 parts by weight of carbon black A-H, 5 parts by weight of a PTFE dispersion having a sulfur content of 20 ppm as the fluororesin and a fluororesin concentration of 50% by mass, 15 parts by weight of a surfactant (TRITON (registered trademark) X-100), and 65 parts by weight of ion-exchanged water in a planetary mixer.

[0032] The microporous layer coating liquid was applied to carbon paper (TGP-H-060: manufactured by Toray Industries, Inc.) that had been treated with the PTFE dispersion to make it water-repellent, and then sintered at 350°C for 20 minutes to produce a gas diffusion electrode substrate product. The sulfuric acid content of the resulting gas diffusion electrode substrate product was 1.0 μg / cm 2 The pH of the produced water at the beginning of aging was 3.7, and the pH of the produced water at the end of aging was 5.0.

[0033] [Comparative Example 1] Carbon black A was used as the carbon black for the microporous layer without heat treatment. refer to A gas diffusion electrode substrate product was produced in the same manner as in Example 1. The sulfuric acid content of the obtained gas diffusion electrode substrate product was 1.8 μg / cm 2 In many cases, the pH of the produced water at the beginning of aging was 3.5, and the pH of the produced water at the end of aging did not reach 5.0.

[0034] [ refer to Example 2] refer to The same procedure was repeated except that carbon black B having a sulfur content of 20 ppm was used instead of carbon black AH in the microporous layer of Example 1, a fluororesin dispersion having a sulfur content of 20 ppm was used as the fluororesin, and sintering was carried out at 420°C for 20 minutes. refer to A gas diffusion electrode substrate product was obtained by the same procedure as in Example 1. The sulfuric acid content of the obtained gas diffusion electrode substrate product was 0.7 μg / cm 2 The pH of the produced water at the beginning of aging was 3.9, and the pH of the produced water at the end of aging was 5.2.

[0035] Comparative Example 2 Except that sintering was carried out at 350 °C for 20 min. refer to A gas diffusion electrode substrate product was obtained by the same procedure as in Example 2. The sulfuric acid content of the obtained gas diffusion electrode substrate product was 1.7 μg / cm 2 The pH of the produced water at the beginning of aging was 3.5, and the pH of the produced water at the end of aging had not yet reached 5.0.

[0036] [Example 3] The water-repellent treatment of carbon paper and the fluororesin used in the microporous layer coating liquid were FEP dispersions with a sulfur content of 3 ppm. refer to A gas diffusion electrode substrate product was obtained by the same procedure as in Example 1. The sulfuric acid content of the obtained gas diffusion electrode substrate product was 0.4 μg / cm 2 The pH of the produced water at the beginning of aging was 4.1, and the pH of the produced water at the end of aging was 5.5.

[0037] [Example 4] A gas diffusion electrode substrate product was obtained in the same manner as in Example 3, except that carbon black B was used as the carbon powder for the microporous layer. The sulfuric acid content of the resulting gas diffusion electrode was 0.1 μg / cm 2 The pH of the produced water at the beginning of aging was 4.7, and the pH of the produced water at the end of aging was 6.0.

[0038] Comparative Example 3 International Publication No. 2015 / 125750 refer to Carbon paper was obtained by the method described in Example 1.

[0039] 5 parts by mass of PTFE was added to 95 parts by mass of carbon paper, and the paper was dried by heating at 100°C for 5 minutes to obtain a thickness of 100 μm and a basis weight of 24 g / m. 2 It was decided.

[0040] A microporous layer was formed using a slit die coater. The microporous layer coating liquid used here was acetylene black (Denka Black (registered trademark) manufactured by Denki Kagaku Kogyo Co., Ltd.), a type of carbon black, PTFE (Polyflon (registered trademark) D-1E manufactured by Daikin Industries, Ltd.) as the fluororesin, TRITON (registered trademark) X-100 manufactured by Nacalai Tesque Inc. as the surfactant, and purified water as the dispersion medium. The microporous layer coating liquid was adjusted to 7.7 parts by mass of acetylene black, 4 parts by mass of PTFE, 14 parts by mass of surfactant, and 74.3 parts by mass of purified water. After applying the microporous layer coating liquid using a die coater, the substrate was held horizontally for 60 seconds, and then heated (sintered) at 120 ° C for 10 minutes and at 380 ° C for 10 minutes to obtain a gas diffusion electrode substrate product. The sulfuric acid content of the resulting gas diffusion electrode substrate product was 2.0 μg / cm 2 The pH of the produced water at the beginning of aging was 3.4, and the pH of the produced water at the end of aging had not yet reached 5.0.

Claims

1. A conductive porous body made of carbon fiber and a microporous layer containing carbon powder provided on at least one surface of the conductive porous body, wherein the sulfuric acid content is 0.5 μg / cm ２ The gas diffusion electrode substrate product is as follows:

2. Sulfuric acid content is 0.2 μg / cm ２ 2. The gas diffusion electrode substrate product of claim 1, wherein:

3. 3. The gas diffusion electrode substrate product according to claim 1, wherein a fluororesin having a sulfur content of 50 ppm or less is adhered to the carbon fiber.

4. 4. The gas diffusion electrode substrate product according to claim 1, wherein the carbon powder has a sulfur content of 3000 ppm or less.

5. 5. The gas diffusion electrode substrate product according to claim 1, wherein the microporous layer comprises a fluororesin having a sulfur content of 50 ppm or less.

6. A polymer electrolyte fuel cell incorporating the gas diffusion electrode substrate product according to any one of claims 1 to 5.