Membrane electrode assembly and fuel cell

Abstract

The present invention addresses the problem of providing a fuel cell membrane electrode assembly that has excellent moisture retention and drainage characteristics and has favorable power generation performance under dry and wet conditions. The present invention is a membrane electrode assembly that has a structure that is formed by layering, in order, an anode-side gas diffusion electrode, an anode catalyst layer, an electrolyte membrane, a cathode catalyst layer, and a cathode-side gas diffusion electrode. The anode-side gas diffusion electrode and the cathode-side gas diffusion electrode include: a conductive porous base material that includes carbon fibers as a constituent material; and a microporous layer that contacts one surface of the conductive porous base material. The anode-side gas diffusion electrode has a lower F / C ratio on the microporous layer side and a lower F / C ratio on the conductive porous base material side than the cathode-side gas diffusion electrode.

Description

Membrane electrode assembly and fuel cell

[0001] The present invention relates to a membrane electrode assembly containing a gas diffusion electrode as a component, used in fuel cells, particularly polymer electrolyte fuel cells, and to a fuel cell having said membrane electrode assembly.

[0002] Solid polymer fuel cells, which generate electromotive force through electrochemical reactions occurring at both electrodes by supplying a hydrogen-containing fuel gas to the anode and an oxygen-containing oxidizing gas to the cathode, are generally composed of a power generation unit called a stack, in which multiple cells are stacked in series. Each cell is a single unit consisting of a separator, gas diffusion electrode, catalyst layer, electrolyte membrane, catalyst layer, gas diffusion electrode, and separator stacked in that order. Here, the three layers of the catalyst layer, electrolyte membrane, and catalyst layer are called a catalyst-coated electrolyte membrane (CCM), and the five layers including the CCM and the gas diffusion electrodes placed on both sides are called a membrane electrode assembly (MEA).

[0003] Gas diffusion electrodes require high gas diffusivity to diffuse the gas supplied from the separator into the catalyst layer, high drainage to discharge the water generated by the electrochemical reaction back to the separator, and high conductivity to extract the generated current. For this reason, gas diffusion electrodes are widely used that utilize a conductive porous substrate made of conductive fibers such as carbon fibers, with a microporous layer (MPL) formed on its surface (Patent Documents 1-3).

[0004] In the operation of a typical fuel cell, the reaction shown in equation (1) proceeds at the anode, and the reaction shown in equation (2) proceeds at the cathode, with different reactions occurring at each side. ２ →2H ＋ +2e － (1) O ２ +4H ＋ +4e － →2H ２ O (2).

[0005] In the operating environment of polymer electrolyte fuel cells, power generation performance decreases under dry conditions (high temperature and low water generation) and wet conditions (low temperature and high water generation). Therefore, gas diffusion electrodes must be able to maintain both moisture retention under dry conditions and drainage under wet conditions. To address these issues, one method is to apply different gas diffusion electrodes to the anode and cathode sides.

[0006] For example, regarding the gas diffusion electrodes used on the anode and cathode sides, a combination has been proposed in which the bending strength and in-plane permeability per unit thickness are greater for the cathode-side gas diffusion electrode than for the anode-side gas diffusion electrode (Patent Document 4).

[0007] Furthermore, a combination has been proposed in which the range of gas diffusion resistance of the gas diffusion electrodes at each electrode is defined, and the gas diffusion resistance of the anode-side gas diffusion electrode is greater than that of the cathode-side gas diffusion electrode (Patent Document 5).

[0008] Japanese Patent Publication No. 2014-011163, International Publication No. 2016 / 076132, International Publication No. 2015 / 125749, Japanese Patent Publication No. 2021-026909, Japanese Patent Publication No. 2022-065838

[0009] However, conventional methods have made it difficult to achieve both moisture retention and drainage properties while improving power generation performance under dry and wet conditions. The present invention aims to provide a membrane electrode assembly for fuel cells that has excellent moisture retention and drainage properties and good power generation performance under dry and wet conditions.

[0010] In order to solve the above problems, the present invention provides the following membrane electrode assembly and fuel cell. (1) A membrane electrode assembly having a structure in which an anode-side gas diffusion electrode, an anode catalyst layer, an electrolyte membrane, a cathode catalyst layer, and a cathode-side gas diffusion electrode are laminated in this order, wherein the anode-side gas diffusion electrode and the cathode-side gas diffusion electrode each have a conductive porous substrate containing carbon fiber as a constituent material and a microporous layer in contact with one surface of the conductive porous substrate, and the anode-side gas diffusion electrode has a smaller F / C ratio on the microporous layer side and a smaller F / C ratio on the conductive porous substrate side than the cathode-side gas diffusion electrode. (2) The membrane electrode assembly according to (1), wherein the ratio of the F / C ratio on the microporous layer side of the cathode-side gas diffusion electrode to the F / C ratio on the microporous layer side of the anode-side gas diffusion electrode is 1.2 or more. (3) The membrane electrode assembly according to (1), wherein the ratio of the F / C ratio on the conductive porous substrate side of the cathode-side gas diffusion electrode to the F / C ratio on the conductive porous substrate side of the anode-side gas diffusion electrode is 1.2 or more. (4) The membrane electrode assembly according to (3), wherein the F / C ratio on the conductive porous substrate side of the anode-side gas diffusion electrode is 0.1 or more and 0.3 or less, and the F / C ratio on the conductive porous substrate side of the cathode-side gas diffusion electrode is 0.3 or more and 0.5 or less. (5) The membrane electrode assembly according to (2), wherein the F / C ratio on the microporous layer side of the anode-side gas diffusion electrode is 0.05 or more and 0.2 or less, and the F / C ratio on the microporous layer side of the cathode-side gas diffusion electrode is 0.2 or more and 0.4 or less. (6) The membrane electrode assembly according to (1), wherein the in-plane air permeability of the anode-side gas diffusion electrode is smaller than the in-plane air permeability of the cathode-side gas diffusion electrode. (7) The in-plane air permeability of the anode-side gas diffusion electrode is 500 μm ３ or more and 850 μm ３ or less, and the in-plane air permeability of the cathode-side gas diffusion electrode is 700 μm ３ or more and 1,150 μm ３The membrane electrode assembly according to (6) below. (8) The membrane electrode assembly according to (1), wherein the thermal resistance of the anode-side gas diffusion electrode is smaller than the thermal resistance of the cathode-side gas diffusion electrode. (9) The membrane electrode assembly according to (8), wherein the difference between the thermal resistance of the anode-side gas diffusion electrode and the thermal resistance of the cathode-side gas diffusion electrode is 0.1 K / W or more. (10) A fuel cell having the membrane electrode assembly according to any one of (1) to (9).

[0011] According to the present invention, it is possible to achieve both moisture retention and drainage, and improve the power generation performance under dry conditions and wet conditions.

[0012] The membrane electrode assembly of the present invention has a structure in which an anode-side gas diffusion electrode, an anode catalyst layer, an electrolyte membrane, a cathode catalyst layer, and a cathode-side gas diffusion electrode are laminated in this order.

[0013] In the membrane electrode assembly of the present invention, the gas diffusion electrode used as the anode-side gas diffusion electrode and the cathode-side gas diffusion electrode has a configuration including a conductive porous substrate and a microporous layer contacting one surface thereof. First, the conductive porous substrate will be described.

[0014] As the conductive porous substrate, for example, a porous substrate containing carbon fibers such as carbon fiber woven fabric, carbon fiber paper, carbon fiber non-woven fabric, carbon felt, carbon paper, and carbon cloth as constituent materials is used. Here, carbon paper refers to a sheet formed by binding a carbon fiber paper with a resin carbide. Among these, since they have excellent corrosion resistance, it is preferable to use, for example, carbon felt, carbon paper, and carbon cloth. Furthermore, since it has excellent characteristics for absorbing dimensional changes in the thickness direction of the electrolyte membrane, that is, "springiness", it is particularly suitable to use carbon paper.

[0015] As the carbon fiber, for example, polyacrylonitrile (PAN)-based, pitch-based, rayon-based, etc. are used. Among them, PAN-based carbon fibers and pitch-based carbon fibers are preferably used because of their excellent mechanical strength. Also, natural fibers and synthetic fibers such as rayon fibers, acrylic fibers, and cellulose fibers may be mixed.

[0016] It is preferable that the average diameter of the single fiber of the carbon fiber is 3 μm or more and 20 μm or less. By setting the average diameter to preferably 3 μm or more, more preferably 5 μm or more, the pore diameter of the conductive porous substrate becomes larger, improving the drainage performance and suppressing flooding. On the other hand, by setting the average diameter to preferably 20 μm or less, more preferably 10 μm or less, the water vapor diffusivity becomes smaller, improving the moisture retention performance and suppressing dry-out.

[0017] It is preferable that the average length of the single fiber of the carbon fiber is 3 mm or more and 20 mm or less. By setting the average length to preferably 3 mm or more, more preferably 5 mm or more, the mechanical strength, conductivity, and thermal conductivity of the conductive porous substrate become good. On the other hand, by setting the average length to preferably 20 mm or less, more preferably 15 mm or less, when using a carbon fiber paper sheet as the conductive porous substrate, the dispersibility of the carbon fiber during papermaking becomes good, and a homogeneous conductive porous substrate can be obtained.

[0018] The mass (basis weight) per unit area of the conductive porous substrate is preferably 20 g / m ２ or more and 50 g / m ２ or less. By setting the basis weight of the conductive porous substrate to preferably 20 g / m ２ or more, more preferably 30 g / m ２ or more, the mechanical strength and conductivity of the conductive porous substrate are improved. On the other hand, by setting the basis weight of the conductive porous substrate to preferably 50 g / m ２ or less, more preferably 40 g / m ２ or less, the gas diffusivity of the conductive porous substrate becomes good, and the power generation performance is improved.

[0019] The adjustment of the basis weight of the conductive porous substrate can be performed by controlling the amount of carbon fiber, resin carbide, etc., which are the constituent materials of the conductive porous substrate.

[0020] The conductive porous substrate is one that has been treated with a water-repellent resin containing fluorine atoms (hereinafter referred to as fluororesin). The water-repellent treatment is preferably carried out using a fluororesin having a fluoroalkyl chain. Examples of fluororesins included in the conductive porous substrate are PTFE (polytetrafluoroethylene), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), ETFE (tetrafluoroethylene-ethylene copolymer), PVDF (polyvinylidene fluoride), and PVF (polyvinyl fluoride), but PTFE or FEP, which exhibit high water repellency, are preferred.

[0021] The amount of fluororesin contained in the conductive porous substrate is not particularly limited, but it is preferably 0.1% by mass or more and 20% by mass or less when the total mass of the conductive porous substrate after water-repellent treatment is taken as 100% by mass. By setting the amount of fluororesin contained in the conductive porous substrate to 0.1% by mass or more, sufficient water repellency is achieved. On the other hand, by setting the amount of fluororesin contained in the conductive porous substrate to 20% by mass or less, it is possible to suppress the fluororesin from blocking pores that serve as gas diffusion pathways or from increasing electrical resistance. Methods for water-repellent treatment of the conductive porous substrate include immersion in a dispersion of fluororesin, as well as applying fluororesin to the conductive porous substrate by die coating, spray coating, etc. Dry processes such as sputtering of fluororesin can also be applied. After water-repellent treatment, a drying process and a heating process to wet and spread the fluororesin within the conductive porous substrate may be added as needed.

[0022] The porosity of the conductive porous substrate is preferably 80% or more and 95% or less. By setting the porosity to preferably 80% or more, and more preferably 85% or more, gas diffusion is increased and power generation performance is improved. On the other hand, by setting the porosity to preferably 95% or less, and more preferably 90% or less, mechanical strength is increased and conductivity is also improved. The porosity of the conductive porous substrate can be measured using a hydrometer or the like.

[0023] In polymer electrolyte fuel cells, the gas diffusion electrode is required to have high gas diffusivity for diffusing the gas supplied from the separator to the catalyst and high drainage capacity for discharging water generated by the electrochemical reaction back to the separator. For this reason, it is preferable that the conductive porous substrate has a pore diameter peak between 10 μm and 100 μm. The pore diameter and its distribution can be determined by measuring the pore diameter distribution using a mercury porosimeter. The pore diameter of the conductive porous substrate can be determined by measuring using only the conductive porous substrate or by measuring using the gas diffusion electrode after the microporous layer has been formed. When measuring using the gas diffusion electrode, the following procedure is followed. First, the structure of each layer is confirmed by scanning electron microscope (SEM) observation of the cross-section perpendicular to the surface (cross-section parallel to the thickness direction) of the gas diffusion electrode, and the approximate pore diameter value of the pore portion of the conductive porous substrate is determined from the SEM image. Next, by comparing the multiple pore diameter peaks obtained by the mercury porosimeter with the approximate pore diameter values ​​obtained by the SEM image described above, only the pore diameter values ​​corresponding to the conductive porous substrate are extracted.

[0024] Next, the microporous layer will be described. The gas diffusion electrode used in the membrane electrode assembly of the present invention has a microporous layer in contact with one surface of the conductive porous substrate. The roles of the microporous layer include moisture management such as moisturizing the electrolyte membrane and draining generated water, reduction of interfacial electrical resistance between the catalyst layer and the gas diffusion electrode, and suppression of damage to the electrolyte membrane by the carbon fibers protruding from the conductive porous substrate. Furthermore, the microporous layer must contain a fluororesin, and preferably contains carbon fine particles.

[0025] As the fluororesin contained in the microporous layer, a fluororesin having a fluoroalkyl chain is preferred in terms of chemical stability and water repellency, and examples of fluororesins suitable for water-repellent treatment of conductive porous substrates include PTFE, FEP, PFA, and ETFE.

[0026] Examples of the carbon nanoparticles include carbon black, carbon nanofibers, carbon nanotubes, and graphene. Among these, inexpensive carbon black is preferably used.

[0027] The basis weight of the aforementioned microporous layer is 10 g / m². ２ 35g / m or more ２ The following is preferable: The basis weight of the microporous layer is preferably 10 g / m². ２ More preferably, 15 g / m ２ By doing so, the carbon fibers protruding from the surface of the conductive porous substrate are covered, thereby preventing the carbon fibers from damaging the adjacent electrolyte membrane. In addition, the contact resistance between the gas diffusion electrode and the catalyst layer can be reduced, and the drying of the electrolyte membrane can be prevented. Furthermore, the basis weight of the microporous layer is preferably 35 g / m². ２ More preferably, 25 g / m ２ The following will improve drainage.

[0028] The thickness of the gas diffusion electrode is preferably 130 μm or more and 190 μm or less. Here, the thickness of the gas diffusion electrode refers to the thickness when both surfaces are sandwiched with a pressure of 0.15 MPa. By setting the thickness of the gas diffusion electrode to 130 μm or more, mechanical strength is maintained and handling in the manufacturing process becomes easier. On the other hand, by setting the thickness of the gas diffusion electrode to 190 μm or less, gas diffusivity is increased and electrical resistance is reduced, thereby improving the power generation performance of the fuel cell. The thickness of the gas diffusion electrode can be adjusted to a suitable range by appropriately adjusting the thickness of the conductive porous substrate and the microporous layer.

[0029] In the membrane electrode assembly of the present invention, the F / C ratio on the conductive porous substrate side of the anode-side gas diffusion electrode is required to be smaller than the F / C ratio on the conductive porous substrate side of the cathode-side gas diffusion electrode. To satisfy this requirement, for example, it is necessary to adjust the amount of fluororesin used for water-repellent treatment of the conductive porous substrate used for each gas diffusion electrode. More specifically, if the conductive porous substrates used for the anode-side and cathode-side gas diffusion electrodes are the same and the type of fluororesin used for water-repellent treatment is also the same, the above requirement can be satisfied by reducing the amount of fluororesin attached to the conductive porous substrate on the anode side compared to the amount of fluororesin attached to the conductive porous substrate on the cathode side.

[0030] The membrane electrode assembly of the present invention requires that the F / C ratio of the microporous layer of the anode-side gas diffusion electrode is smaller than the F / C ratio of the microporous layer of the cathode-side gas diffusion electrode. To satisfy this requirement, it is necessary to adjust, for example, the amount of fluororesin used in the microporous layer for each gas diffusion electrode. More specifically, if the composition of the fluororesin and other constituent materials used in the microporous layers for the anode-side and cathode-side gas diffusion electrodes is the same, the above requirement can be satisfied by using less fluororesin in the microporous layer of the anode-side than in the microporous layer of the cathode-side.

[0031] Here, the F / C ratio refers to the ratio of the mass concentration of fluorine to the mass concentration of carbon on each surface, and the F / C ratio on the conductive porous substrate side can be determined as follows. First, several locations (for example, 5 locations) are randomly selected from the surface of the conductive porous substrate side of the gas diffusion electrode, and a scanning electron microscope (SEM)-energy dispersive X-ray analysis (EDX) system is used to scan a predetermined area (for example, 600 μm × 400 μm) and measure the mass concentrations (%) of fluorine (F) and carbon (C). The measurement conditions are, for example, an acceleration voltage of 7 kV and a magnification of 200x. From the obtained measured values, the F / C ratio can be calculated by dividing the mass concentration (%) of F by the mass concentration (%) of C, and then calculating the average value for the selected locations. The F / C ratio on the microporous layer side can also be determined from the surface of the microporous layer side using the same method as the measurement method for the F / C ratio on the conductive porous substrate side.

[0032] Generally speaking, not only with gas diffusion electrodes, but when the F / C ratio is high, i.e., when the amount of fluorine is high, the water-repellent property increases, and when the F / C ratio is low, i.e., when the amount of fluorine is low, the hydrophilic property, which is the property of adsorbing water, increases. Here, regarding the relationship between the F / C ratio of the gas diffusion electrodes on the anode and cathode sides and the cell state during power generation, although not limited to theory, it can be considered as follows. First, if the F / C ratio of the gas diffusion electrodes at both electrodes is the same, under dry conditions, the CCM near the anode catalyst layer where water is not generated dries out and its performance deteriorates (dry-out), and under wet conditions, water accumulates in the CCM and gas diffusion electrodes near the cathode catalyst layer where water is generated, and its performance deteriorates (flooding). On the other hand, if the F / C ratio of the gas diffusion electrode on the anode side is smaller than the F / C ratio of the gas diffusion electrode on the cathode side, the reverse diffusion phenomenon in which water generated in the cathode catalyst layer flows toward the anode side becomes larger, so it is presumed that the dry-out and flooding mentioned above can be suppressed.

[0033] The relative magnitudes of the F / C ratios of the anode-side gas diffusion electrode and the cathode-side gas diffusion electrode can be determined by comparing the F / C ratios of the conductive porous substrate side and the microporous layer side of the gas diffusion electrode.

[0034] As described above, in the film electrode assembly of the present invention, the F / C ratio of the anode-side gas diffusion electrode on the conductive porous substrate side is set to be smaller than the F / C ratio of the cathode-side gas diffusion electrode on the conductive porous substrate side, and the F / C ratio of the anode-side gas diffusion electrode on the microporous layer side is set to be smaller than the F / C ratio of the cathode-side gas diffusion electrode on the microporous layer side.

[0035] In the membrane electrode assembly of the present invention, the ratio (B / A) of the F / C ratio (B) of the cathode-side gas diffusion electrode to the F / C ratio (A) of the microporous layer side of the anode-side gas diffusion electrode is preferably 1.2 or more and 10 or less. Furthermore, the ratio (D / C) of the F / C ratio (D) of the cathode-side gas diffusion electrode to the F / C ratio (C) of the conductive porous substrate side of the anode-side gas diffusion electrode is preferably 1.2 or more and 10 or less.

[0036] By setting B / A and D / C to 1.2 or higher, power generation performance under both dry and wet conditions is improved. Although this cannot be fully explained by theory alone, the inventors have found the following through diligent research: When the F / C ratio of the gas diffusion electrodes at both poles is the same (B / A = 1 and D / C = 1), under dry conditions, the CCM near the anode catalyst layer where water is not generated dries out, reducing power generation performance (dry-out), and under wet conditions, water accumulates in the CCM and gas diffusion electrodes near the cathode catalyst layer where water is generated, reducing power generation performance (flooding). On the other hand, when the F / C ratio of the anode-side gas diffusion electrode is smaller than the F / C ratio of the cathode-side gas diffusion electrode (B / A > 1 and D / C > 1), the reverse diffusion phenomenon in which water generated in the cathode catalyst layer flows toward the anode side becomes larger, which is presumed to suppress the dry-out and flooding mentioned above. Here, by setting B / A to preferably 1.2 or higher, more preferably 1.5 or higher, the moisture retention on the anode side is increased and dry-out can be sufficiently suppressed, and by setting D / C to preferably 1.2 or higher, more preferably 1.5 or higher, the drainage on the cathode side is increased and flooding can be sufficiently suppressed.

[0037] Furthermore, by setting B / A and D / C to 10 or less, it is possible to suppress the increase in electrical resistance caused by an excess of fluororesin in the cathode-side gas diffusion electrode.

[0038] In the membrane electrode assembly of the present invention, it is preferable that the F / C ratio (C) on the conductive porous substrate side of the anode-side gas diffusion electrode is 0.1 or more and 0.3 or less, and the F / C ratio (D) on the conductive porous substrate side of the cathode-side gas diffusion electrode is 0.3 or more and 0.5 or less. Furthermore, in the membrane electrode assembly of the present invention, it is preferable that the F / C ratio (A) on the microporous layer side of the anode-side gas diffusion electrode is 0.05 or more and 0.2 or less, and the F / C ratio (B) on the microporous layer side of the cathode-side gas diffusion electrode is 0.2 or more and 0.4 or less. By setting C to 0.1 or more and D to 0.3 or more and 0.5 or less, and A to 0.05 or more and B to 0.2 or more and 0.4 or less, power generation performance is improved under both dry and wet conditions.

[0039] In the film electrode assembly of the present invention, it is preferable that the in-plane permeability of the anode-side gas diffusion electrode is smaller than the in-plane permeability of the cathode-side gas diffusion electrode. It is preferable to make the in-plane permeability of the anode-side gas diffusion electrode smaller than the in-plane permeability of the cathode-side gas diffusion electrode, and more preferably the in-plane permeability of the anode-side gas diffusion electrode 500 to 850 μm. ３ Furthermore, the in-plane air permeability of the cathode-side gas diffusion electrode is set to 700 to 1,150 μm. ３ Under these conditions, by making the in-plane permeability of the anode-side gas diffusion electrode smaller than that of the cathode-side gas diffusion electrode, power generation performance under both dry and wet conditions can be improved.

[0040] Although this cannot be fully explained by theory alone, the inventors, after diligent research, have come to the following conclusions regarding the reverse diffusion phenomenon in which water generated in the cathode catalyst layer flows to the anode side, as described above. First, under dry conditions, it is presumed that because the in-plane permeability of the anode-side gas diffusion electrode is smaller than that of the cathode-side gas diffusion electrode, water that reaches the anode side diffuses further through the anode-side gas diffusion electrode, thereby suppressing dry-out. Next, under wet conditions, it is presumed that because the in-plane permeability of the cathode-side gas diffusion electrode is larger than that of the anode-side gas diffusion electrode, water generated in the cathode catalyst layer diffuses more easily through the cathode-side gas diffusion electrode, thereby suppressing flooding.

[0041] The in-plane air permeability of a gas diffusion electrode can be measured, for example, by the following method. First, a donut-shaped piece of the gas diffusion electrode (e.g., outer diameter 40 mm, inner diameter 12 mm) is cut out as a sample and placed between two flat plate fixtures attached to a precision universal testing machine (e.g., Shimadzu Corporation's "Autograph®" AGS-X). Here, a hole for injecting pressurized air from the outside is provided in the center of the upper fixture, and air is introduced into the donut-shaped sample sandwiched between the upper and lower fixtures from the inner side of the sample, passes through the plane of the sample, and exits from the outer side of the sample. Furthermore, a regulator and flow meter are provided upstream of the incoming airflow path, allowing the pressure to be set to an arbitrary value and the flow rate to be measured simultaneously.

[0042] When a sample is held between two layers of air at a predetermined pressure (e.g., 1 MPa), and the air pressure is P (kPa) and the flow rate is Q (L / min), the in-plane air permeability of the sample (μm) can be calculated from the rate of change (ΔQ / ΔP) using the following formula. ３ The in-plane permeability (μm) can be determined. Measurements are taken for five samples, and the average value is taken as the in-plane permeability of the gas diffusion electrode. ３ )=(ΔQ / ΔP)×(1.8 / (2×60×π))×(ln(40 / 12))×10 ７ ···(formula).

[0043] In the film electrode assembly of the present invention, it is preferable that the electrical resistance of the anode-side gas diffusion electrode is smaller than the electrical resistance of the cathode-side gas diffusion electrode. It is preferable to make the electrical resistance of the anode-side gas diffusion electrode smaller than the electrical resistance of the cathode-side gas diffusion electrode, and more preferably the electrical resistance of the anode-side gas diffusion electrode is 5.0 to 7.0 mΩ·cm. ２ Furthermore, the electrical resistance of the cathode-side gas diffusion electrode is set to 6.0 to 8.5 mΩ·cm. ２ Under these conditions, by making the electrical resistance of the anode-side gas diffusion electrode smaller than that of the cathode-side gas diffusion electrode, power generation performance under both dry and wet conditions can be improved.

[0044] Although this cannot be fully explained by theory alone, the inventors, after diligent research, have come to the following conclusions regarding the situation in which a reverse diffusion phenomenon occurs where water generated in the cathode catalyst layer flows to the anode side, as described above. First, under dry conditions, it is presumed that because the electrical resistance of the anode-side gas diffusion electrode is lower than that of the cathode-side gas diffusion electrode, the generation of Joule heat on the anode side during power generation is small, suppressing the temperature rise and thus preventing dry-out caused by the drying of water that reaches the anode side. Next, under wet conditions, it is presumed that because the electrical resistance of the cathode-side gas diffusion electrode is higher than that of the anode-side gas diffusion electrode, the generation of Joule heat on the cathode side during power generation is large, promoting a temperature rise and accelerating the drying of water generated in the cathode catalyst layer, thereby suppressing flooding.

[0045] The electrical resistance of a gas diffusion electrode can be measured, for example, by the following method. First, a sample of the gas diffusion electrode is cut to a predetermined size (e.g., 22.4 mm square) and placed between two flat plate fixtures mounted on a precision universal testing machine (e.g., Shimadzu Corporation's "Autograph®" AGS-X). The flat plate fixtures are conductive and rigid (e.g., made of stainless steel), and the surface in contact with the sample is treated to further enhance conductivity (e.g., gold plating). Two terminals of a digital multimeter are connected to the upper and lower fixtures, respectively. With the sample held at a predetermined pressure (e.g., 1 MPa), the electrical resistance is read when a predetermined DC current (e.g., 1 A) is applied using a digital multimeter, and the measured value (mΩ) is multiplied by the measurement area (cm²). ２ By multiplying by ), the value of electrical resistance per unit area (mΩ・cm) is obtained. ２ The following is calculated: Measure five samples and use the average value as the electrical resistance of the gas diffusion electrode.

[0046] In the film electrode assembly of the present invention, it is preferable that the thermal resistance of the anode-side gas diffusion electrode is smaller than the thermal resistance of the cathode-side gas diffusion electrode. By making the thermal resistance of the anode-side gas diffusion electrode smaller than the thermal resistance of the cathode-side gas diffusion electrode, and more preferably by making the difference between the thermal resistance of the anode-side gas diffusion electrode and the thermal resistance of the cathode-side gas diffusion electrode 0.1 K / W or more (making the difference obtained by subtracting the thermal resistance of the anode-side gas diffusion electrode from the thermal resistance of the cathode-side gas diffusion electrode 0.1 K / W or more), power generation performance under dry and wet conditions can be improved.

[0047] As described above, under conditions where a reverse diffusion phenomenon occurs in which water generated in the cathode catalyst layer flows to the anode side, it is presumed that by making the electrical resistance of the anode-side gas diffusion electrode lower than the electrical resistance of the cathode-side gas diffusion electrode, the effect is achieved based on the proportional relationship between electrical resistance and thermal resistance in conductors, as follows: That is, under dry conditions, as the electrical resistance of the anode-side gas diffusion electrode becomes lower than the electrical resistance of the cathode-side gas diffusion electrode, the thermal resistance of the anode-side gas diffusion electrode becomes lower than the thermal resistance of the cathode-side gas diffusion electrode, and it is presumed that dry-out is suppressed according to the mechanism described above. Also, under wet conditions, as the electrical resistance of the cathode-side gas diffusion electrode becomes higher than the electrical resistance of the anode-side gas diffusion electrode, the thermal resistance of the cathode-side gas diffusion electrode becomes higher than the thermal resistance of the anode-side gas diffusion electrode, and it is presumed that flooding is suppressed according to the mechanism described above.

[0048] The thermal resistance of a gas diffusion electrode can be measured by the following method, for example: Cut a test piece of a predetermined size (e.g., φ30 mm) from the gas diffusion electrode and set it in a thermal resistance measuring device. Cool one side and heat the other side, and pressurize it with a predetermined surface pressure (e.g., 1 MPa). Calculate the thermal resistance value (K / W) from the heat flow rate and the temperature difference between the test pieces. As a thermal resistance measuring device, for example, a thermal resistance measuring device (IE-1230) manufactured by Iwatsu Instruments Co., Ltd. can be used.

[0049] Next, an example of a method for manufacturing a gas diffusion electrode according to the present invention will be described, in which the microporous layer is formed on one surface of the conductive porous substrate. However, the method for manufacturing a gas diffusion electrode according to the present invention is not limited to the description below.

[0050] First, a method for manufacturing the conductive porous substrate will be described.

[0051] A carbon fiber paper is manufactured by loosening bundles of carbon fibers cut to a predetermined length in water, uniformly dispersing them to produce a carbon fiber dispersion, forming it into a paper roll, and drying it. Here, a water-soluble resin such as polyvinyl alcohol or polyvinyl acetate may be included to maintain the shape of the carbon fiber paper.

[0052] Next, in order to improve the mechanical strength of the conductive porous substrate and reduce electrical resistance, the intersections of the carbon fibers are bonded together with a resin carbide.

[0053] To bond the intersections of carbon fibers with resin carbides, a preferred method involves impregnating a carbon fiber papermaking body with a resin composition solution, followed by heat treatment to carbonize the resin components in the resin composition.

[0054] Examples of resin components used in resin composition solutions include thermosetting resins such as phenolic resins, epoxy resins, melamine resins, and furan resins. In addition to the resin components and solvent, the resin composition solution may also contain carbon powder, surfactants, etc. Examples of carbon powders include carbon black, graphite, carbon nanotubes, and carbon nanofibers.

[0055] Methods for impregnating with a resin composition solution include, for example, immersion, spray application, blade coating, die coating, and transfer methods.

[0056] Next, the carbon fiber paper body impregnated with the resin composition solution (hereinafter referred to as the resin-impregnated body) is heated in air (at a temperature of, for example, 80 to 150°C) and dried. Subsequently, by further heating in air at a higher temperature (for example, 200 to 300°C), the thermosetting resin is cured while surfactants and other substances are decomposed and removed. At this time, the flatness and thickness may be adjusted by heating and pressing both sides of the resin-impregnated body with a hot plate.

[0057] Furthermore, in order to enhance the conductivity and long-term durability of the conductive porous substrate, it is preferable to heat the resin composition after curing in an inert atmosphere such as nitrogen (at a temperature of, for example, 1,000 to 2,400°C). This process yields a carbon fiber paper, i.e., carbon paper, in which the intersections of carbon fibers are bonded together with resin carbides.

[0058] Next, to improve the drainage properties of the obtained carbon paper, a water-repellent treatment with a fluororesin is performed. Methods for water-repellent treatment of carbon paper include, for example, immersing the carbon paper in a dispersion of fluororesin, or applying fluororesin to the carbon paper by die coating, spray coating, etc. After the water-repellent treatment, a drying process and a heating process to wet and spread the fluororesin throughout the carbon paper are added as needed to obtain a conductive porous substrate.

[0059] Next, a method for forming the microporous layer on one surface of the conductive porous substrate will be described.

[0060] The aforementioned microporous layer can be formed by applying a coating solution for forming a microporous layer, which contains carbon fine particles and fluororesin dispersed in a dispersion medium such as water, to one surface of a conductive porous substrate and then performing a heat treatment.

[0061] First, carbon nanoparticles and fluororesin are mixed with a dispersion medium such as water, and then kneaded using a homogenizer, planetary mixer, ultrasonic disperser, etc., to obtain a coating liquid for forming a microporous layer.

[0062] When preparing the coating solution for forming the microporous layer, it is preferable to add a dispersant or thickener to the solution, as this enhances the dispersion stability of carbon nanoparticles and fluororesin. Examples of dispersants include polyoxyethylene octylphenyl ether-based "Triton®" X-100 (manufactured by Nacalai Tesque Co., Ltd.), which is a nonionic surfactant with a low metal component content.

[0063] Methods for applying the coating solution for forming the microporous layer to one surface of a conductive porous substrate include, for example, screen printing, gravure printing, spray coating, die coating, bar coating, blade coating, and roll knife coating using various commercially available coating devices.

[0064] After applying the microporous layer-forming coating liquid to one surface of the conductive porous substrate, it is heated (for example, at a temperature of 60 to 150°C) to dry it, and then heated to an even higher temperature (for example, 250 to 380°C) to decompose and remove additives such as dispersants and thickeners, and to promote the melting of the fluororesin, thereby obtaining a gas diffusion electrode in which a microporous layer is formed on one surface of the conductive porous substrate.

[0065] A membrane electrode assembly (MEA) can be fabricated by joining the surface of the microporous layer of the anode gas diffusion electrode to the surface of the anode catalyst layer of an electrolyte membrane (CCM) having catalyst layers on both sides, and by joining the surface of the microporous layer of a separately prepared cathode gas diffusion electrode to the surface of the cathode catalyst layer on the opposite side of the CCM. Furthermore, a fuel cell can be fabricated by sandwiching the obtained MEA from both sides with two separators, one for the anode and one for the cathode, each having a gas flow path. In this case, the MEA is made to be slightly smaller than the outer circumference of the two separators, and a gasket is placed between the MEA and the separators so as to surround the outside of the MEA. When the MEA is compressed and sandwiched between the separators under pressure, the MEA is fixed in a state of being compressed to an appropriate thickness. By changing the thickness of the gasket to adjust it to the thickness of the MEA in the power generation cell, performance that matches the specifications required for the fuel cell (for power generation purposes) can be achieved.

[0066] The separator's flow path is equipped with an inlet and outlet for supplying hydrogen on the anode side, and an inlet and outlet for supplying air on the cathode side. Furthermore, since the separator is made of conductive materials such as stainless steel or carbon, electricity can be supplied and discharged by connecting electrical wiring. Additionally, by providing a passage for supplying circulating water inside the separator, the cell can be maintained at a predetermined temperature.

[0067] By preparing approximately 200 to 500 sets of the aforementioned fuel cell cells and connecting them in series, a fuel cell stack capable of generating high voltages such as 40V to 200V can be manufactured. Such a fuel cell stack can be used as a power source for fuel cell vehicles and the like. The fuel cell of the present invention refers to a fuel cell cell having the aforementioned membrane electrode assembly, and a fuel cell stack formed by connecting multiple sets of these fuel cell cells.

[0068] Next, embodiments of the present invention will be specifically described by reference to the following examples. It should be noted that the present invention is not limited to the embodiments described below. First, the evaluation methods used in the examples are shown below.

[0069] <Measurement of the basis weight of the gas diffusion electrode> Cut a 10 cm square piece of the gas diffusion electrode to make a sample, and measure its mass [g] over the area of ​​the sample (0.01 m²). ２ This was obtained by dividing by ().

[0070] <Measurement of F / C Ratio of Gas Diffusion Electrode> The F / C ratio of the gas diffusion electrode on the conductive porous substrate side or the microporous layer side was measured by the following method. Five locations were randomly selected from the surface of the gas diffusion electrode to be measured, and a scanning electron microscope (SEM)-energy dispersive X-ray analysis (EDX) system was used to scan a 600 μm × 400 μm area. The mass concentrations (%) of fluorine (F) and carbon (C) were measured under measurement conditions of an acceleration voltage of 7 kV and a magnification of 200x. For the five obtained measurements, the individual F / C ratio was calculated by dividing the mass concentration (%) of F by the mass concentration (%) of C, and the average value of the five F / C ratios was calculated and taken as the F / C ratio of the gas diffusion electrode.

[0071] <Measurement of In-Plane Air Permeability of Gas Diffusion Electrodes> Five donut-shaped samples of gas diffusion electrodes, each with an outer diameter of 40 mm and an inner diameter of 12 mm, were prepared. One sample was placed between two flat plate fixtures, one above the other, attached to a Shimadzu Corporation "Autograph®" AGS-X. The in-plane air permeability was measured five times (once per sample x 5 samples). Specifically, air was introduced from the inner side of the donut-shaped sample, passed through the plane of the sample, and exited from the outer side of the sample, sandwiched between the lower flat plate fixture and the upper flat plate fixture, which had a hole in the center for injecting pressurized air from the outside. A regulator and flow meter were placed upstream of the airflow path, and the flow rate was measured while the pressure was set to an arbitrary level.

[0072] When a sample is sandwiched between 1 MPa of air, and the air pressure is P (kPa) and the flow rate is Q (L / min), the in-plane air permeability (μm) of each sample can be calculated from the rate of change (ΔQ / ΔP) using the following formula. ３ The value of the five measured values ​​was calculated. The average value of the five values ​​was then determined and taken as the in-plane air permeability of the gas diffusion electrode. In-plane air permeability (μm ３ )=(ΔQ / ΔP)×(1.8 / (2×60×π))×(ln(40 / 12))×10 ７ ···(formula).

[0073] <Measurement of Electrical Resistance of Gas Diffusion Electrodes> Five samples of gas diffusion electrodes, each cut to 22.4 mm square, were prepared and placed between the two flat plate fixtures of a Shimadzu Corporation "Autograph®" AGS-X. The electrical resistance was measured five times (once per sample x 5 samples). Specifically, the sample was placed in a SUS (stainless steel) flat plate fixture with gold plating on the surface in contact with the sample, and two terminals of a digital multimeter were connected to it, one at the top and one at the bottom. With the sample clamped at 1 MPa, the electrical resistance was read when a current of 1 A was applied using a digital multimeter, and the measured value (mΩ) was recorded over the measurement area (cm²). ２ By multiplying by ), the electrical resistance value per unit area (mΩ·cm²) for each sample can be calculated. ２ The following was calculated: The average of the five measured values ​​was then determined and used as the electrical resistance of the gas diffusion electrode.

[0074] <Measurement of Thermal Resistance of Gas Diffusion Electrode> Five samples of gas diffusion electrode, each cut to a diameter of 30 mm, were prepared and set in a thermal resistance measuring device (IE-1230, manufactured by Iwatsu Instruments Co., Ltd.). One side was cooled and the other side was heated, and under pressure of 1 MPa, the thermal resistance value (K / W) was calculated for each sample from the heat flow rate and the temperature difference between the test pieces. The average value of the five measured values ​​was then calculated and defined as the thermal resistance of the gas diffusion electrode.

[0075] <Evaluation of power generation performance> A catalyst solution was prepared by sequentially adding 1 g of platinum-supported carbon (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., platinum load: 50% by mass), 1 g of purified water, 8 g of "Nafion®" solution (5% by mass of "Nafion®" manufactured by Sigma-Aldrich), and 18 g of isopropyl alcohol (manufactured by Nacalai Tesque Co., Ltd.).

[0076] Next, a 50mm x 50mm piece of "NAFRONT®" PTFE tape "TOMBO®" No. 9001 (manufactured by Nichias Corporation) was cut and coated with a catalyst solution by spraying. It was then dried at room temperature until the platinum content reached 0.3 mg / cm². ２ A PTFE sheet with a catalyst layer was prepared. Next, a solid polymer electrolyte membrane (film thickness: 10 μm) was prepared using a 10% by mass dispersion of "Nafion®" (manufactured by Sigma-Aldrich) and cut to 80 mm x 80 mm. The obtained solid polymer electrolyte membrane was sandwiched between two PTFE sheets with catalyst layers and pressed at 5 MPa under pressure at 130°C for 5 minutes using a flat plate press to transfer the catalyst layer to the solid polymer electrolyte membrane. After pressing, the PTFE sheets were peeled off to prepare a solid polymer electrolyte membrane with catalyst layers on both sides.

[0077] Next, a solid polymer electrolyte membrane having catalyst layers on both sides was sandwiched between two gas diffusion electrodes (detailed conditions described later), each cut to 50 mm x 50 mm, with the side containing the microporous layer in contact with the catalyst layer. The electrodes were then pressed in a flat plate press at 1 MPa and 130°C for 5 minutes to produce a membrane electrode assembly.

[0078] The obtained membrane electrode assembly was incorporated into the "JARI Standard Cell" (electrode area: 5 cm x 5 cm), a single cell for fuel cell evaluation developed and standardized by the Japan Automobile Research Institute (JARI), and the voltage was measured when the current density was changed. In addition, unpressurized hydrogen was supplied to the anode side and unpressurized air to the cathode side for evaluation.

[0079] The fuel cell temperature was set to 80°C, and the supplied hydrogen and air were humidified using a humidifier pot set to 60°C, resulting in a humidity of 80% within the cell. The utilization rates of hydrogen and oxygen in the air were 70% and 40%, respectively. Current density: 0.5 A / cm² ２ and 2.0 A / cm ２ The output voltage was measured and used as an indicator of dry performance (moisture retention) and wet performance (drainage), respectively.

[0080] <Manufacturing Method for Carbon Paper A> Toray Industries, Inc.'s polyacrylonitrile-based carbon fiber "Torayca (registered trademark)" T300 (average fiber diameter: 7 μm) is cut to a length of 6 mm, dispersed in water, and continuously paper-making is performed. Furthermore, a 10% by mass aqueous solution of polyvinyl alcohol is spray-coated and then dried, resulting in a basis weight of 30 g / m². ２ A carbon fiber paper was obtained. The amount of polyvinyl alcohol attached to the carbon fiber paper was 18 parts by mass per 100 parts by mass of carbon fiber.

[0081] Next, a resin composition solution was prepared by mixing flake graphite (average particle size: 5 μm), phenolic resin (a mixture of resol-type phenolic resin and novolac-type phenolic resin in a mass ratio of 1:1), and methanol in a mass ratio of 3:17:80.

[0082] Next, the carbon fiber paper mass described above was cut into 20 cm squares and immersed in a resin composition solution filling a tank. The amount of resin composition adhering to the mass was then adjusted by sandwiching it between two metal rolls. At this time, a certain clearance was left between the two rolls, and the total amount of resin composition adhering was adjusted by passing the immersed carbon fiber paper mass between them. The amount of resin composition adhering was adjusted so that the total amount of phenolic resin and flake graphite was 80 parts by mass for every 100 parts by mass of carbon fiber in the carbonized carbon paper described later. After that, it was placed on a hot plate at 100°C and heated for 5 minutes to dry, thereby producing a resin-impregnated body.

[0083] Next, the obtained resin-impregnated material was sandwiched between two parallel hot plates and subjected to a heat and pressure treatment at 180°C for 5 minutes. Release paper was placed between the resin-impregnated material and the hot plates to prevent adhesion, and spacers were placed around the edges of the upper and lower hot plates to adjust the thickness. Afterward, the material was heated in a furnace under a nitrogen atmosphere at 2,400°C to carbonize it and obtain carbon paper A. The obtained carbon paper had a thickness of 150 μm and a basis weight of 44 g / m² at 0.15 MPa. ２ That was the case.

[0084] <Manufacturing Method for Carbon Paper B> Toray Industries, Inc.'s polyacrylonitrile-based carbon fiber "Torayca (registered trademark)" T300 (average fiber diameter: 7 μm) is cut to a length of 6 mm, dispersed in water, and continuously paper-making is performed. Furthermore, a 10% by mass aqueous solution of polyvinyl alcohol is spray-coated and then dried, resulting in a basis weight of 24 g / m². ２ A carbon fiber paper was obtained. The amount of polyvinyl alcohol attached to the carbon fiber paper was 18 parts by mass per 100 parts by mass of carbon fiber.

[0085] Next, a resin composition solution was prepared by mixing flake graphite (average particle size: 5 μm), phenolic resin (a mixture of resol-type phenolic resin and novolac-type phenolic resin in a mass ratio of 1:1), and methanol in a mass ratio of 15:15:70.

[0086] Next, the carbon fiber paper mass described above was cut into 20 cm squares and immersed in a resin composition solution filling a tank. The amount of resin composition adhering to the mass was then adjusted by sandwiching it between two metal rolls. At this time, a certain clearance was left between the two rolls, and the total amount of resin composition adhering was adjusted by passing the immersed carbon fiber paper mass between them. The amount of resin composition adhering was adjusted so that the total amount of phenolic resin and flake graphite was 150 parts by mass for every 100 parts by mass of carbon fiber in the carbonized carbon paper described later. After that, it was placed on a hot plate at 100°C and heated for 5 minutes to dry, thereby producing a resin-impregnated body.

[0087] Next, the obtained resin-impregnated material was sandwiched between parallel hot plates and subjected to a heat and pressure treatment at 180°C for 5 minutes. Release paper was placed between the resin-impregnated material and the hot plates to prevent adhesion, and spacers were placed around the edges of the upper and lower hot plates to adjust the thickness. Afterward, the material was heated in a furnace under a nitrogen atmosphere at 2,400°C to carbonize it and obtain carbon paper B. The obtained carbon paper had a thickness of 130 μm and a basis weight of 49 g / m² at 0.15 MPa. ２ That was the case.

[0088] <Manufacturing Method for Carbon Paper C> Toray Industries, Inc.'s polyacrylonitrile-based carbon fiber "Torayca (registered trademark)" T300 (average fiber diameter: 7 μm) is cut to a length of 6 mm, dispersed in water, and continuously paper-making is performed. Furthermore, a 10% by mass aqueous solution of polyvinyl alcohol is spray-coated and then dried, resulting in a basis weight of 30 g / m². ２ A carbon fiber paper was obtained. The amount of polyvinyl alcohol attached to the carbon fiber paper was 18 parts by mass per 100 parts by mass of carbon fiber.

[0089] Next, a resin composition solution was prepared by mixing flake graphite (average particle size: 5 μm), phenolic resin (a mixture of resol-type phenolic resin and novolac-type phenolic resin in a mass ratio of 1:1), and methanol in a mass ratio of 2:18:80.

[0090] Next, the carbon fiber paper mass described above was cut into 20 cm squares and immersed in a resin composition solution filling a tank. The amount of resin composition adhering to the mass was then adjusted by sandwiching it between two metal rolls. At this time, a certain clearance was left between the two rolls, and the total amount of resin composition adhering was adjusted by passing the immersed carbon fiber paper mass between them. The amount of resin composition adhering was adjusted so that the total amount of phenolic resin and flake graphite was 60 parts by mass for every 100 parts by mass of carbon fiber in the carbonized carbon paper described later. After that, it was placed on a hot plate at 100°C and heated for 5 minutes to dry, thereby producing a resin-impregnated body.

[0091] Next, the obtained resin-impregnated material was sandwiched between parallel hot plates and subjected to a heat and pressure treatment at 180°C for 5 minutes. Release paper was placed between the resin-impregnated material and the hot plates to prevent adhesion, and spacers were placed around the periphery of the upper and lower hot plates to adjust the thickness. Afterward, the material was heated in a furnace under a nitrogen atmosphere at 2,400°C to carbonize it and obtain carbon paper C. The obtained carbon paper had a thickness of 160 μm and a basis weight of 39 g / m² at 0.15 MPa. ２ That was the case.

[0092] (Example 1) The carbon paper A described above was impregnated with a 3% by mass fluororesin dispersion prepared by mixing a dispersion of PTFE fine particles, "Polyflon®" D-210C (manufactured by Daikin Industries, Ltd., PTFE fine particle concentration 60%), with ion-exchanged water as a water-repellent treatment. After impregnation, it was dried at 100°C for 5 minutes to obtain a conductive porous substrate. Before drying after impregnation, the carbon paper A was passed between two metal rolls with a certain clearance between them to obtain a fluororesin adhesion amount of 1 g / m². ２ The material was adjusted to achieve this, and a conductive porous substrate was obtained.

[0093] A coating solution for forming a microporous layer was prepared using "Denka Black" powder (manufactured by Denka Co., Ltd.) as carbon nanoparticles, PTFE as fluororesin, "Triton" X-100 (manufactured by Nakalai Tesque Co., Ltd.) as a dispersant, and water as a dispersion medium. The mixing ratio was set to carbon nanoparticles / fluororesin / dispersant = 75 parts by mass / 8 parts by mass / 150 parts by mass, so that the undegradable components (carbon nanoparticles and fluororesin) accounted for 23% by mass of the total amount. In this process, "Polyflon" D-210C (manufactured by Daikin Industries, Ltd.), a dispersion of PTFE particles dispersed in water, was used as the source of PTFE. Furthermore, when preparing the coating solution for forming a microporous layer, a planetary mixer was used to disperse the raw materials so that the coating solution composition was uniform.

[0094] On one side of the conductive porous substrate described above, the basis weight after heating using a die coater is 15 g / m². ２ The above-mentioned microporous layer-forming coating liquid was applied while adjusting the conditions to achieve the desired result, dried at 100°C for 10 minutes, and then heated at 350°C for 10 minutes to promote adhesion between the fluororesin and carbon nanoparticles while decomposing and removing dispersants, etc., thereby forming a microporous layer. This created a gas diffusion electrode (anode-side gas diffusion electrode) having a microporous layer on one side of a conductive porous substrate.

[0095] The obtained gas diffusion electrode had a thickness of 168 μm, with an F / C ratio of 0.08 on the microporous layer side and 0.18 on the conductive porous substrate side. The electrical resistance of the gas diffusion electrode was 6.4 mΩ·cm. ２ The in-plane air permeability is 790 μm. ３ That was the case.

[0096] Next, the amount of fluororesin applied was 2.5 g / m ２ A conductive porous substrate was obtained by performing a water-repellent treatment on carbon paper A in the same manner as described above, except that the other condition was as described above. Furthermore, a coating liquid for forming a microporous layer was prepared in the same manner as described above, except that the mixing ratio was carbon fine particles:fluororesin:dispersant = 75 parts by mass:25 parts by mass:150 parts by mass, and the coating liquid for forming a microporous layer was applied to one side of the conductive porous substrate to fabricate a gas diffusion electrode (cathode-side gas diffusion electrode).

[0097] The obtained gas diffusion electrode had a thickness of 176 μm, with an F / C ratio of 0.32 on the microporous layer side and an F / C ratio of 0.39 on the conductive porous substrate side. The electrical resistance of the gas diffusion electrode was 8.0 mΩ·cm. ２ The in-plane air permeability is 980 μm. ３ That was the case.

[0098] Furthermore, the F / C ratio of the cathode-side gas diffusion electrode to the anode-side gas diffusion electrode was 4.00 on the microporous layer side and 2.17 on the conductive porous substrate side.

[0099] Using the anode-side gas diffusion electrode and cathode-side gas diffusion electrode described above, a membrane electrode assembly was fabricated using the method described above, and a fuel cell was then fabricated using the method described above. When the power generation performance was evaluated, the dry performance was 0.77 V and the wet performance was 0.63 V.

[0100] (Example 2) An anode-side gas diffusion electrode, a cathode-side gas diffusion electrode, a membrane electrode assembly, and a fuel cell cell were fabricated in the same manner as in Example 1, except that the above-described carbon paper B was used on the anode and cathode sides to create a conductive porous substrate. The results are shown in Table 1.

[0101] (Example 3) An anode-side gas diffusion electrode, a cathode-side gas diffusion electrode, a membrane electrode assembly, and a fuel cell cell were fabricated in the same manner as in Example 1, except that the above-mentioned carbon paper C was used on the anode and cathode sides to create a conductive porous substrate. The results are shown in Table 1.

[0102] (Example 4) The basis weight of the microporous layer in the anode gas diffusion electrode was set to 25 g / m². ２ Except for the changes made, the anode-side gas diffusion electrode, cathode-side gas diffusion electrode, membrane electrode assembly, and fuel cell were fabricated in the same manner as in Example 2. The results are shown in Table 1.

[0103] (Example 5) The basis weight of the microporous layer in the cathode-side gas diffusion electrode was set to 25 g / m². ２ Except for the changes made, the anode-side gas diffusion electrode, cathode-side gas diffusion electrode, membrane electrode assembly, and fuel cell were fabricated in the same manner as in Example 2. The results are shown in Table 1.

[0104] (Example 6) An anode gas diffusion electrode, a cathode gas diffusion electrode, a membrane electrode assembly, and a fuel cell cell were fabricated in the same manner as in Example 1, except that the mixing ratio of the coating liquid for forming a microporous layer in the cathode gas diffusion electrode was set to carbon fine particles:fluororesin:dispersant = 75 parts by mass:30 parts by mass:150 parts by mass. The results are shown in Table 1.

[0105] (Example 7) In the water-repellent treatment of carbon paper constituting the conductive porous substrate of the anode-side gas diffusion electrode, the amount of fluororesin deposited was 1.5 g / m². ２ The adjustment is made so that the amount of fluororesin deposited on the carbon paper constituting the conductive porous substrate of the cathode-side gas diffusion electrode is 2 g / m². ２ Except for adjusting the settings to achieve the desired result, the anode-side gas diffusion electrode, cathode-side gas diffusion electrode, membrane electrode assembly, and fuel cell were fabricated in the same manner as in Example 1. The results are shown in Table 1.

[0106] (Example 8) An anode gas diffusion electrode, a cathode gas diffusion electrode, a membrane electrode assembly, and a fuel cell cell were fabricated in the same manner as in Example 7, except that the mixing ratio of the coating liquid for forming a microporous layer in the anode gas diffusion electrode was set to carbon fine particles:fluororesin:dispersant = 75 parts by mass:15 parts by mass:150 parts by mass. The results are shown in Table 1.

[0107] (Example 9) An anode gas diffusion electrode, a cathode gas diffusion electrode, a membrane electrode assembly, and a fuel cell cell were fabricated in the same manner as in Example 3, except that the mixing ratio of the coating liquid for forming a microporous layer in the anode gas diffusion electrode was set to carbon fine particles:fluororesin:dispersant = 105 parts by mass:11 parts by mass:120 parts by mass. The results are shown in Table 1.

[0108] (Example 10) The mixing ratio of the coating solution for forming a microporous layer on the cathode-side gas diffusion electrode was set to carbon fine particles:fluororesin:dispersant = 100 parts by mass:50 parts by mass:50 parts by mass, and the basis weight of the microporous layer was 20 g / m². ２ Except for the changes made, the anode-side gas diffusion electrode, cathode-side gas diffusion electrode, membrane electrode assembly, and fuel cell were fabricated in the same manner as in Example 3. The results are shown in Table 1.

[0109] (Comparative Example 1) An anode gas diffusion electrode, a cathode gas diffusion electrode, a membrane electrode assembly, and a fuel cell were fabricated in the same manner as in Example 1, except that the anode gas diffusion electrode of Example 1 was also used as the cathode gas diffusion electrode. The results are shown in Table 1.

[0110] (Comparative Example 2) An anode gas diffusion electrode, a cathode gas diffusion electrode, a membrane electrode assembly, and a fuel cell were fabricated in the same manner as in Example 1, except that the cathode gas diffusion electrode of Example 1 was also used as the anode gas diffusion electrode. The results are shown in Table 1.

[0111] (Comparative Example 3) An anode gas diffusion electrode, a cathode gas diffusion electrode, a membrane electrode assembly, and a fuel cell cell were fabricated in the same manner as in Example 2, except that the mixing ratio of the coating liquid for forming a microporous layer in the anode gas diffusion electrode was set to carbon fine particles:fluororesin:dispersant = 75 parts by mass:25 parts by mass:150 parts by mass. The results are shown in Table 1.

[0112] (Comparative Example 4) In the water-repellent treatment of carbon paper constituting the conductive porous substrate of the anode-side gas diffusion electrode, the amount of fluororesin deposited was 2.5 g / m². ２ Except for adjusting the settings accordingly, the anode-side gas diffusion electrode, cathode-side gas diffusion electrode, membrane electrode assembly, and fuel cell were fabricated in the same manner as in Example 3. The results are shown in Table 1.

[0113] (Comparative Example 5) In the water-repellent treatment of carbon paper constituting the conductive porous substrate of the anode-side gas diffusion electrode, the amount of fluororesin attached was 2 g / m². ２ The mixture is adjusted to the following: the mixing ratio of the coating solution for forming the microporous layer on the anode gas diffusion electrode is set to carbon nanoparticles:fluororesin:dispersant = 100 parts by mass:60 parts by mass:100 parts by mass, and the basis weight of the microporous layer is 25 g / m². ２ Except for the changes made, the anode-side gas diffusion electrode, cathode-side gas diffusion electrode, membrane electrode assembly, and fuel cell were fabricated in the same manner as in Example 3. The results are shown in Table 1.

[0114] (Comparative Example 6) An anode gas diffusion electrode, a cathode gas diffusion electrode, a membrane electrode assembly, and a fuel cell were fabricated in the same manner as in Example 1, except that the cathode gas diffusion electrode of Example 1 was used as the anode gas diffusion electrode, and the anode gas diffusion electrode of Example 1 was used as the cathode gas diffusion electrode. The results are shown in Table 1.

[0115]

[0116] The membrane electrode assembly of the present invention can be suitably used in fuel cells, particularly solid polymer fuel cells used as power sources for mobile vehicles such as cars, ships, railways, and aircraft.

Claims

1. A membrane electrode assembly having a structure in which an anode gas diffusion electrode, an anode catalyst layer, an electrolyte membrane, a cathode catalyst layer, and a cathode gas diffusion electrode are stacked in this order, wherein the anode gas diffusion electrode and the cathode gas diffusion electrode each have a conductive porous substrate containing carbon fibers as a constituent material and a microporous layer in contact with one surface of the conductive porous substrate, and the anode gas diffusion electrode has a smaller F / C ratio on the microporous layer side and a smaller F / C ratio on the conductive porous substrate side than the cathode gas diffusion electrode.

2. The membrane electrode assembly according to claim 1, wherein the ratio of the F / C ratio on the microporous layer side of the cathode gas diffusion electrode to the F / C ratio on the microporous layer side of the anode gas diffusion electrode is 1.2 or more.

3. The film electrode assembly according to claim 1, wherein the ratio of the F / C ratio of the cathode-side gas diffusion electrode on the conductive porous substrate side to the F / C ratio of the anode-side gas diffusion electrode on the conductive porous substrate side is 1.2 or more.

4. The membrane electrode assembly according to claim 3, wherein the F / C ratio on the conductive porous substrate side of the anode-side gas diffusion electrode is 0.1 or more and 0.3 or less, and the F / C ratio on the conductive porous substrate side of the cathode-side gas diffusion electrode is 0.3 or more and 0.5 or less.

5. The membrane electrode assembly according to claim 2, wherein the F / C ratio on the microporous layer side of the anode-side gas diffusion electrode is 0.05 or more and 0.2 or less, and the F / C ratio on the microporous layer side of the cathode-side gas diffusion electrode is 0.2 or more and 0.4 or less.

6. The film electrode assembly according to claim 1, wherein the in-plane permeability of the anode-side gas diffusion electrode is smaller than the in-plane permeability of the cathode-side gas diffusion electrode.

7. The in-plane air permeability of the anode-side gas diffusion electrode is 500 μm. ３ 850 μm or more ３ The following conditions apply, and the in-plane air permeability of the cathode-side gas diffusion electrode is 700 μm. ３ 1,150 μm or more ３ The membrane electrode assembly according to claim 6, which is as follows:

8. The film electrode assembly according to claim 1, wherein the thermal resistance of the anode-side gas diffusion electrode is smaller than the thermal resistance of the cathode-side gas diffusion electrode.

9. The film electrode assembly according to claim 8, wherein the difference between the thermal resistance of the anode-side gas diffusion electrode and the thermal resistance of the cathode-side gas diffusion electrode is 0.1 K / W or more.

10. A fuel cell having a membrane electrode assembly according to any one of claims 1 to 9.

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