Electrode substrate and method for manufacturing the same

Abstract

The purpose of the present invention is to provide an electrode base material which achieves a good balance among high drainage performance, high gas diffusibility and spring properties, while maintaining adequate electrical conductivity.　In order to achieve the above-described purpose, an electrode base material according to the present invention is obtained by filling voids of a carbon fiber sheet with a fluororesin and a fine carbon powder; with respect to the fluorine / carbon ratio (F / C) thereof, among the average (F / C-1) in one surface, the average (F / C-2) in the center plane in the thickness direction, and the average (F / C-3) in the other surface, the ratio of the minimum value (F / Cmin) to the maximum value (F / Cmax) is 0.80 or more; the pore size distribution thereof has at least a first peak that is present within the pore diameter range of not less than 0.5 μm but less than 120 μm, and a second peak that is present within the pore diameter range of not less than 10 nm but less than 0.5 μm; and the ratio of the peak volume (V1) of the first peak to the peak volume (V2) of the second peak, namely V1 / V2 is from 1.0 to 7.0.

Description

[Technical Field]

[0001] This invention relates to an electrode substrate and a method for manufacturing the same. [Background technology]

[0002] In recent years, fuel cells that use hydrogen as fuel have attracted attention as an energy source that does not emit carbon dioxide. A fuel cell is a type of power generation device that supplies a fuel gas containing hydrogen to the anode and an oxidizing gas containing oxygen to the cathode, and generates an electromotive force through an electrochemical reaction that occurs at both electrodes.

[0003] Among fuel cells, polymer electrolyte fuel cells (MSF) are particularly versatile. A single cell of a MSF is generally constructed by stacking a separator, a gas diffusion electrode substrate, a catalyst layer, an electrolyte membrane, another catalyst layer, a gas diffusion electrode substrate, and a separator in that order. Of these, the gas diffusion electrode substrate needs to have gas diffusion properties to diffuse the gas supplied from the separator to the catalyst layer, drainage properties to discharge water generated by the electrochemical reaction to the separator, and conductivity to extract the generated current.

[0004] To achieve both conductivity and drainage, a method is used in which conductive carbon fine powder, such as carbon black, and a highly water-repellent fluororesin are attached to the gas diffusion electrode substrate. For example, Patent Document 1 proposes a method in which fluororesin and carbon fine powder are applied to one side of a glass fiber nonwoven fabric, and the density is increased by hot pressing, thereby achieving both drainage and conductivity in the gas diffusion electrode substrate. Patent Document 2 improves conductivity by adding short-length carbon fibers to a gas diffusion electrode substrate mainly composed of fluororesin and carbon fine powder. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2010-192350 [Patent Document 2] International Publication No. 2010 / 050219 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The gas diffusion electrode substrate described in Patent Document 1 increases conductivity while suppressing deflection by increasing the density of the gas diffusion electrode substrate. However, this reduces the porosity within the substrate, resulting in insufficient gas diffusion.

[0007] The gas diffusion electrode substrate described in Patent Document 2 is mainly composed of fluororesin and carbon fine powder. Due to the low springiness in the thickness direction of the substrate, it cannot absorb dimensional changes in the electrolyte membrane, raising concerns that excessive pressure may be applied to the electrolyte membrane.

[0008] The object of the present invention is to provide an electrode substrate that maintains conductivity while simultaneously achieving high drainage, gas diffusion, and spring properties. [Means for solving the problem]

[0009] The electrode substrate of the present invention, which solves the above problems, is an electrode substrate in which a fluororesin and carbon fine powder are filled in the voids of a carbon fiber sheet, The minimum value (F / C) among the average value (F / C-1) on one surface, the average value (F / C-2) on the center plane in the thickness direction, and the average value (F / C-3) on the other surface, measured by scanning electron microscopy-energy dispersive X-ray spectroscopy. min ) Maximum value (F / C max The ratio to ) is 0.80 or higher, Furthermore, the pore size distribution measured by the mercury intrusion method has at least one first peak in the range of pore diameters from 0.5 μm to less than 120 μm, and a second peak in the range of 10 nm to less than 0.5 μm, and the ratio (V1 / V2) of the peak volume of the first peak (V1) to the peak volume of the second peak (V2) is between 1.0 and 7.0.

[0010] Further, the method for manufacturing the electrode substrate of the present invention includes an immersion step of immersing a carbon fiber sheet in a dispersion liquid containing a fluororesin and carbon fine powder, a heat treatment step for melting the fluororesin contained in the dispersion liquid immersed in the immersion step, and has.

Effects of the Invention

[0011] By using the electrode substrate of the present invention for a gas diffusion electrode, it is possible to achieve both conductivity, drainage, gas diffusion, and springiness.

Embodiments for Carrying Out the Invention

[0012] <于 Hereinafter, the present invention will be described in detail. Since the electrode substrate of the present invention is particularly preferably used as a gas diffusion electrode of a solid polymer fuel cell, in this specification, as an effect when the electrode substrate takes a specific aspect, the effect when used as a gas diffusion electrode may be described. Further, in this specification, "~" shall mean a numerical range including the boundary values of its upper and lower limits.

[0013] <Electrode Substrate> The "carbon fiber sheet" in this specification is a structure having a porous structure composed of a network structure essentially composed of carbon fibers and voids between the carbon fibers. Specific examples of the carbon fiber sheet include a carbon fiber paper sheet, a woven fabric, and a non-woven fabric. Among them, since it is excellent in the property of absorbing dimensional changes in the plane direction of the electrolyte membrane, that is, "springiness", it is particularly preferable that the carbon fiber sheet is a carbon fiber paper sheet. Here, the carbon fiber paper sheet refers to a structure in which carbon fibers are randomly dispersed in a two-dimensional plane.

[0014] The carbon fiber paper can be produced by a wet papermaking method in which carbon fibers are dispersed in a liquid or a dry papermaking method in which carbon fibers are dispersed in air. Among them, the wet papermaking method is preferably used because of its excellent productivity. The carbon fiber paper may be formed by mixing carbon powder or organic fibers for papermaking in order to improve the conductivity and drainage of the gas diffusion electrode substrate. In addition, since the form retention and handling properties are easily improved, it may be formed by using an organic polymer such as polyvinyl alcohol, polyvinyl acetate, polyacrylonitrile, or cellulose as a binder.

[0015] Not limited to the case of the carbon fiber paper, the carbon fiber sheet in the present invention may contain a binder for binding carbon fibers to each other. On the other hand, it is preferable that the electrode substrate of the present invention does not contain a thermosetting resin as a binder for binding the carbon fibers constituting the carbon fiber sheet, and it is more preferable that the binder consists only of a thermoplastic resin. By not containing a thermosetting resin as a binder, there is an advantage that it becomes easy to separate, recover, and reuse the carbon fibers after performing the baking-out treatment of the binder.

[0016] The carbon fibers constituting the carbon fiber sheet are not particularly limited, and examples thereof include polyacrylonitrile (PAN)-based, pitch-based, and rayon-based carbon fibers. Among them, PAN-based carbon fibers or pitch-based carbon fibers are preferably used because of their excellent mechanical strength.

[0017] In the case of the carbon fiber papermaking body, which is the most preferred embodiment, the average diameter of the individual carbon fibers (hereinafter referred to as "carbon fiber diameter") is preferably in the range of 3 to 20 μm, and more preferably in the range of 5 to 10 μm. When the carbon fiber diameter is 3 μm or more, more preferably 5 μm or more, the diameter of the voids becomes larger, resulting in good drainage. On the other hand, when the carbon fiber diameter is 20 μm or less, more preferably 10 μm or less, the thickness unevenness becomes smaller, making it easier to control the thickness within the preferred electrode substrate thickness range described later. The carbon fiber diameter is determined by taking photographs of the carbon fibers magnified 1000 times using a microscope such as a scanning electron microscope, randomly selecting 30 individual fibers, and measuring their diameters to obtain the average value.

[0018] Furthermore, in the case of carbon fiber papermaking, the average fiber length of the individual carbon fibers constituting the carbon fiber papermaking body (hereinafter referred to as "carbon fiber length") is preferably in the range of 5 to 20 mm, and more preferably in the range of 5 to 15 mm. When the carbon fiber length is 5 mm or more, the carbon fiber papermaking body tends to have excellent springiness, mechanical strength, electrical conductivity, and thermal conductivity. On the other hand, when the carbon fiber length is 20 mm or less, more preferably 15 mm or less, the dispersion of carbon fibers during papermaking is excellent, and a homogeneous carbon fiber papermaking body is more easily obtained. The carbon fiber length is determined by taking photographs of carbon fibers magnified 50 times using a microscope such as a scanning electron microscope, randomly selecting 30 individual fibers, and measuring their lengths to obtain the average value.

[0019] The electrode substrate of the present invention is formed by filling the voids of a carbon fiber sheet with fluororesin and carbon fine powder, and the minimum value (F / C) of the average value of the fluorine / carbon ratio (F / C) measured by scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX analysis) on one surface (F / C-1), the average value on the center surface in the thickness direction (F / C-2), and the average value on the other surface (F / C-3) is obtained. min ) Maximum value (F / C maxThe ratio to ) is 0.80 or higher, more preferably 0.85 or higher. The average F / C values ​​F / C-1 to F / C-3 on each surface can be measured by the method described in the examples below. When F / C-1 to F / C-3 satisfy the above relationship, it means that in the electrode substrate of the present invention, the fluororesin and carbon fine powder are filled substantially uniformly in the thickness direction in the voids of the carbon fiber sheet. Therefore, in this specification, when F / C-1 to F / C-3 satisfy the above relationship, it may be expressed as the fluororesin and carbon fine powder being uniform in the thickness direction. When the fluororesin is uniform in the thickness direction, liquid water is less likely to accumulate in the electrode substrate, resulting in an electrode substrate with excellent drainage. Furthermore, when the carbon fine powder is uniform in the thickness direction, conductive paths are uniformly formed in the electrode substrate, resulting in an electrode substrate with excellent conductivity. Minimum value (F / C min ) Maximum value (F / C max As a method for setting the ratio to the above range, for example, one method is to immerse the carbon fiber sheet in a dispersion containing fluororesin and carbon fine powder, thereby simultaneously bringing the fluororesin and carbon fine powder into contact with the carbon fiber sheet, as in the method for manufacturing the electrode substrate of the present invention described later. A more preferred method is to ensure that a sufficient amount of carbon fine powder is present in the dispersion relative to the fluororesin.

[0020] As the fluororesin, one or more resins selected from the group consisting of polytetrafluoroethylene (PTFE), tetrafluoroethylene hexafluoropropylene copolymer (FEP), perfluoroalkoxy fluororesin (PFA), ethylene tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), and polyvinyl fluoride (PVF) are used, with PTFE or FEP being preferred as they exhibit strong water repellency.

[0021] In the electrode substrate of the present invention, the fluororesin content is preferably 10% by weight or more, and more preferably 15% by weight or more. By setting the fluororesin content to 10% by weight or more, and more preferably 15% by weight or more, the mechanical strength of the electrode substrate is maintained and high drainage properties are achieved, enabling excellent power generation performance in the low-temperature, high-current-density range. On the other hand, the fluororesin content is preferably 40% by weight or less, and more preferably 30% by weight or less. By setting the fluororesin content to 40% by weight or less, and more preferably 30% by weight or less, the conductive resistance of the electrode substrate can be kept low, resulting in even better power generation performance.

[0022] As the carbon fine powder, it is preferable to use carbon black such as furnace black, acetylene black, lamp black, and thermal black. Furthermore, in order to obtain pores with a diameter of 10 nm to 0.5 μm, it is more preferable to use carbon black with an average primary particle diameter of 25 to 60 nm, and even more preferable to use carbon black with an average primary particle diameter of 25 to 50 nm. More specifically, carbon black that has a structural structure in which primary particles are linked together like beads is preferably used, and pores with a diameter of 10 nm to 0.5 μm are formed by the entanglement of this structural structure and binding with fluororesin. In addition, nanocarbon materials such as carbon nanotubes and carbon nanofibers can also be used as the carbon fine powder. Graphite such as flake graphite, scaly graphite, clay graphite, artificial graphite, expanded graphite, and thin-leaf graphite can also be used in combination with carbon black or nanocarbon materials. By adding graphite, which has higher conductivity than nanocarbon materials, the conductivity of the electrode substrate can be further improved.

[0023] In the electrode substrate of the present invention, the carbon fine powder content is preferably 40% by weight or more and 70% by weight or less, and more preferably 45% by weight or more and 60% by weight or less. By having a carbon fine powder content of 40% by weight or more, more preferably 45% by weight or more, the conductivity of the electrode substrate can be increased. On the other hand, by having a carbon fine powder content of 70% by weight or less, the pore volume of 0.5 μm to 120 μm in diameter, which is necessary for gas diffusion, increases, and gas diffusion tends to improve.

[0024] Although fluororesins have low conductivity, the presence of a predetermined amount of carbon fine powder in the mixture allows the carbon fine powder to act as a conductive pathway, thereby imparting sufficient conductivity to the electrode substrate. In the electrode substrate of the present invention, conductivity is ensured by the carbon fine powder contained together with the fluororesin, eliminating the need for the conventional high-temperature firing process exceeding 1000°C, which was used to carbonize the binding resin of the carbon fibers constituting the carbon fiber sheet, thus reducing manufacturing costs.

[0025] More specifically, the electrode substrate of the present invention has at least a first peak in its pore size distribution that is in the range of pore diameters of 0.5 μm or more and less than 120 μm, and a second peak that is in the range of 10 nm or more and less than 0.5 μm, and the ratio of the peak volume of the first peak (V1) to the peak volume of the second peak (V2) (V1 / V2) is between 1.0 and 7.0. The first peak mainly originates from the voids between carbon fibers constituting the carbon fiber sheet, and the second peak mainly originates from the pores of the carbon fine powder. If V1 / V2 is greater than 7.0, the pore size inside the electrode substrate becomes larger, the density of the conductive path made of carbon fine powder decreases, and the conductivity of the electrode substrate deteriorates. By setting V1 / V2 to 7.0 or less, water vapor can be retained near the electrolyte membrane, and the electrolyte membrane can be kept moist. On the other hand, if V1 / V2 is less than 1.0, the pore volume through which the fuel gas passes becomes smaller, and the gas diffusivity decreases. One method for setting V1 / V2 within the above range is to adjust the content of carbon fine powder in the electrode substrate. Specifically, by increasing the content of carbon fine powder, pores derived from the carbon fine powder are formed, and the volume of these pores corresponds to V2. In addition, by increasing the content of carbon fine powder, pores with a volume corresponding to V1 are filled. Therefore, V1 / V2 can be reduced by increasing the content of carbon fine powder. Conversely, V1 / V2 can be increased by decreasing the content of carbon fine powder.

[0026] The electrode substrate of the present invention preferably contains 200 to 400 parts by weight of carbon fine powder per 100 parts by weight of fluororesin, and more preferably 250 to 350 parts by weight. By including 200 parts by weight or more, more preferably 250 parts by weight or more, of carbon fine powder per 100 parts by weight of fluororesin, the conductivity of the electrode substrate can be improved. Furthermore, by including 400 parts by weight or less, more preferably 350 parts by weight or less, of carbon fine powder per 100 parts by weight of fluororesin, it becomes easier to uniformly distribute the carbon fine powder within the electrode substrate.

[0027] For the same reasons as described above for the binder, in the electrode substrate of the present invention, it is preferable not to contain a thermosetting resin as a component filled in the voids of the carbon fiber sheet. That is, when the electrode substrate is calcined in an air atmosphere at 500 °C for 1 hour, the carbon fibers constituting the carbon fiber sheet are not sufficiently bonded to each other, and it is preferable that the carbon fibers can be separated and recovered. Among them, it is more preferable that the binder filled in the voids of the carbon fiber sheet consists only of a thermoplastic resin.

[0028] When the electrode substrate is subjected to a calcination treatment, the binder and fluororesin that fixed the carbon fibers are burned off. Therefore, when the thermosetting resin is not included, the thickness of the electrode substrate increases. The electrode substrate of the present invention preferably has an unpressurized thickness after being calcined in an air atmosphere at 500 °C for 1 hour that is 1.5 times or more, more preferably 2 times or more, the unpressurized thickness before the calcination treatment. The upper limit is not particularly limited, but is usually about 5 times. Such a change in thickness indicates that the carbon fibers constituting the electrode substrate are not sufficiently bonded to each other, and the carbon fibers can be separated and recovered. The unpressurized thickness can be measured by the method described in the examples below.

[0029] The thickness of the electrode substrate is preferably 50 to 230 μm, more preferably 70 to 180 μm. When the thickness is 230 μm or less, more preferably 180 μm or less, the conductivity is high and the generated water is easily discharged. Furthermore, the size of the entire fuel cell can be easily reduced. On the other hand, when the thickness is 50 μm or more, more preferably 70 μm or more, the gas diffusion in the in-plane direction inside is efficiently performed, and the power generation performance is easily improved.

[0030] The density of the electrode substrate is preferably 3 0.50 g / cm or less, more preferably 3 0.40 g / cm or less. When the density is 3 0.50 g / cm or less, more preferably 3As a result of the following, the gas diffusivity is increased, making it easier for liquid water to be discharged into the flow path, thus improving power generation performance, and in addition, the springiness is also improved. On the other hand, the density of the electrode substrate is 0.20 g / cm³. 3 Preferably, it should be 0.25 g / cm³ or more. 3 It is more preferable that the density of the electrode substrate is 0.20 g / cm³. 3 More preferably 0.25 g / cm³ 3 As a result of the above, the mechanical strength is increased and handling becomes easier.

[0031] In the electrode substrate of the present invention, the short-circuit current density of the carbon fiber sheet is 80 mA / cm². 2 The following is preferable: 50 mA / cm² 2 The following is more preferable: Short-circuit current density of 80 mA / cm² 2 More preferably, 50 mA / cm² 2 The following conditions make it less likely for a short circuit to occur in the electrolyte membrane within the fuel cell during power generation, thus preventing a sudden voltage drop. The short-circuit current density of the carbon fiber sheet is preferably as low as possible, ideally 0 mA / cm². 2 This is the most preferable condition. As a method for setting the short-circuit current density of the carbon fiber sheet within the above range, for example, a method of smoothing the surface of the carbon fiber sheet by employing the molding process described later.

[0032] <Method for manufacturing electrode substrates> The present invention preferably provides a method for manufacturing an electrode substrate that includes an immersion step of immersing a carbon fiber sheet in a dispersion containing a fluororesin and carbon fine powder, and a heat treatment step of melting the fluororesin contained in the dispersion immersed in the immersion step.

[0033] [Soaking process] In the immersion process, the carbon fiber sheet is immersed in a dispersion containing fluororesin and carbon fine powder. Please refer to the above description for details on the carbon fiber sheet, fluororesin, and carbon fine powder.

[0034] One method for impregnating carbon fiber sheets with a dispersion is to coat the carbon fiber sheet with the dispersion. However, in the case of coating, it is unavoidable that the fluororesin and carbon fine powder contained in the dispersion will be more abundant on the coated side and less abundant on the other side. Even if heat treatment such as hot pressing is performed after coating, the fluororesin will melt and move to some extent from the coated side to the other side, but the carbon fine powder will not melt even when heated and will not move easily, making it difficult to distribute it uniformly across the entire electrode substrate. By immersing the carbon fiber sheet in a dispersion containing fluororesin and carbon fine powder, it becomes possible to distribute the carbon fine powder uniformly on both sides of the electrode substrate.

[0035] [Molding process] The method for manufacturing the electrode substrate of the present invention preferably includes a molding step in which the substrate is heated and pressurized at a temperature of 100°C or higher after the immersion step and before the heat treatment step. That is, it is also preferable to mold the carbon fiber sheet, which has been immersed in fluororesin and carbon fine powder as described above, by heating and pressurizing it after the immersion step and before the heat treatment step. This step is referred to as the molding step in this specification. This molding step allows for more precise control of the thickness and porosity of the resulting electrode substrate, and the surface of the carbon fiber sheet becomes smooth as the fluororesin on the surface of the carbon fiber sheet partially melts while being pressurized. The aggregate of fluororesin and carbon fine powder can cause a short circuit by piercing the electrolyte membrane as a protrusion when the electrolyte membrane is sandwiched between the electrode substrates for assembly of a fuel cell. This short circuit can be significantly suppressed by smoothing the surface of the carbon fiber sheet. In the molding step, the heating temperature is preferably 100 to 250°C. The applied pressure is preferably 0.01 to 5 MPa.

[0036] [Heat treatment process] Carbon fiber sheets, immersed in a dispersion containing fluororesin and carbon fine powder, and subjected to a molding process as necessary, are then subjected to a heat treatment process. The heat treatment temperature is preferably between 300°C and 400°C. Heat treatment at a temperature of 300°C or higher melts and spreads the fluororesin contained in the impregnated dispersion, imparting water repellency to the entire electrode substrate. Furthermore, by keeping the temperature below 400°C, a large-scale heating furnace becomes unnecessary, significantly reducing the manufacturing cost of the electrode substrate.

[0037] In the method for manufacturing an electrode substrate of the present invention, it is preferable that the content of carbon fine powder be 40% by weight or more and 70% by weight or less of the total weight of the electrode substrate. The reasons why the carbon fine powder content in the electrode substrate is preferably within the above range, and more preferred ranges, are as described above. As a method for achieving the above range of carbon fine powder content in the electrode substrate, for example, one can include adding the amount necessary to achieve the above range of carbon fine powder content in the electrode substrate to the dispersion liquid in the immersion step.

[0038] In the method for manufacturing an electrode substrate of the present invention, it is preferable that the electrode substrate contains carbon fine powder in an amount of 200 to 400 parts by weight per 100 parts by weight of fluororesin. The reasons why the relative content of carbon fine powder and fluororesin in the electrode substrate is preferable to be within the above range, and the more preferable range, are as described above. As a method for making the relative content of carbon fine powder and fluororesin in the electrode substrate within the above range, for example, one method is to make the relative content of fluororesin in the dispersion liquid during the immersion process greater than the relative content in the electrode substrate. Normally, the content of carbon fine powder hardly decreases even in the heat treatment process, whereas the content of fluororesin decreases significantly in the heat treatment process. Therefore, by making the dispersion liquid contain more fluororesin than the relative content in the target electrode substrate, the relative content in the electrode substrate can be made within the above range. Since the degree of reduction of fluororesin in the heat treatment process varies depending on the heat treatment conditions, the content of fluororesin in the dispersion liquid is adjusted as appropriate according to the heat treatment conditions. [Examples]

[0039] Next, the electrode substrate of the present invention will be specifically described with reference to examples, but the electrode substrate of the present invention is not limited to these examples. The materials used in the examples, the method for manufacturing and evaluating the electrode substrate, and the method for evaluating the battery performance of the fuel cell are shown below.

[0040] <Pore size distribution> Electrode substrate 6cm 2 Five measurement samples were prepared by cutting the material into sections. The average peak diameter of the pore size distribution obtained by measuring the pore size distribution using the mercury intrusion method within the measurement pressure range of 6 kPa to 123 MPa (pore size 10 nm to 200 μm) was determined. In cases where multiple peaks appeared in a similar pore size range within a single sample, the peak diameter of the highest peak was adopted. A Shimadzu Autopore 9520 was used as the measurement device. Here, the pore size peak was defined as the maximum point (or the largest maximum point if there are multiple maximum points) in the target region of the logarithmic differential pore volume distribution graph obtained by pore measurement using a mercury porosimeter, and its position was defined as the pore size.

[0041] Furthermore, for the peak pore volume, the cumulative data obtained by integrating the logarithmic differential pore volume distribution graph with respect to pore diameter is calculated by subtracting the cumulative volume data for smaller pore diameters from the cumulative volume value for larger pore diameters of the pores being measured, dividing by the weight of the measured sample, and then taking the average of the values ​​for 5 samples. For example, the peak volume in the second region (the region between 10 nm and 0.5 μm) is obtained by subtracting the cumulative data for less than 10 nm from the cumulative data for less than 0.5 μm, and then dividing by the sample weight.

[0042] <Thickness and density of electrode substrate> The thickness of the electrode substrate was measured using a Nikon Corporation digital thickness gauge "DigiMicro" while applying a load of 0.15 MPa to the electrode substrate. The density was also calculated from the weight and area of ​​the electrode substrate.

[0043] <Fluorine / Carbon ratio (F / C)> [Average F / C ratio on the surface of the electrode substrate (F / C-1, F / C-3)] SEM-EDX analysis was performed under conditions of an acceleration voltage of 10kV and a magnification of 400x. Five elemental mapping images of carbon and fluorine were acquired from both the electrode substrate surface and the other surface, and the ratio of the average signal intensity of fluorine to the average signal intensity of carbon was calculated. A Hitachi S-5500 scanning electron microscope was used, and a Horiba EX-220SE energy-dispersive X-ray analyzer was used.

[0044] [Average F / C ratio (F / C-2) at the center plane in the thickness direction of the electrode substrate] The electrode substrate was halved while being cooled using a cryo-ion beam cross-sectioning apparatus, exposing the surface near the midpoint of the thickness. Then, following the same thickness measurement procedure as described above, the thickness was measured at one point to confirm that the thickness of the electrode substrate after cross-sectioning was half the thickness of the electrode substrate before processing. The F / C ratio was then calculated in the same manner as the measurement of the electrode substrate surface.

[0045] <Unpressurized thickness> Measurements were taken using a Keyence VR-3000 one-shot 3D measuring microscope. An electrode substrate cut to 30mm x 30mm was placed on a stainless steel plate, and the stainless steel plate and electrode substrate were set up so that the stainless steel plate occupied one half of the observation field and the electrode substrate occupied the other half. 3D measurement was performed at a magnification of 12x. The average height of the stainless steel plate area and the electrode substrate area in the obtained image were calculated, and the difference between the average height of the stainless steel plate area and the average height of the electrode substrate area was defined as the unpressurized thickness of the electrode substrate. The unpressurized thickness was similarly measured for the electrode substrate after heating in an air atmosphere at 500°C for 1 hour as a burn-off treatment.

[0046] <Conductive Resistance> The conductive resistance of the electrode substrate in the direction perpendicular to the surface was determined by cutting the electrode substrate to 30 mm x 30 mm, sandwiching it between two gold-plated plates, applying a uniform surface pressure of 1.0 MPa, and passing a current of 1.0 A through it. The resistance at that time was measured and multiplied by the area of ​​the electrode substrate.

[0047] <Springiness> The springiness of the electrode substrate in the direction perpendicular to the surface was calculated by cutting the electrode substrate to 30 mm x 30 mm, sandwiching it between two gold-plated plates, measuring the thickness when a uniform surface pressure of 1.0 MPa was applied, and then measuring the thickness when a uniform surface pressure of 2.0 MPa was applied, and calculating the springiness using the following formula. A smaller springiness value indicates better springiness.

[0048] (Sprungness) = (Thickness at 2.0 MPa) / (Thickness at 1.0 MPa) <Gas diffusibility> Using a water vapor gas permeation diffusion evaluation device (MVDP-200C) manufactured by Seika Sangyo, the gas whose diffusivity was to be measured was flowed through one side (primary side) of the electrode substrate, and nitrogen gas was flowed through the other side (secondary side). The differential pressure between the primary and secondary sides was controlled to be near 0 Pa (0 ± 3 Pa) (i.e., there was almost no gas flow due to the pressure difference, and gas movement occurred only by molecular diffusion), and the gas concentration when equilibrium was reached was measured using a gas concentration meter on the secondary side, and this value (%) was used as an index of gas diffusivity in the thickness direction.

[0049] <Short-circuit current density> A polymer electrolyte membrane "Nafion" (registered trademark) NR212 (manufactured by DuPont) (film thickness 50 μm) was sandwiched between two fabricated electrode substrates. Here, the electrode substrate was a 4 cm square, and the polymer electrolyte membrane was a square of 6 cm or more on each side. The sides of the polymer electrolyte membrane and the electrode substrate were aligned parallel to each other, and the centers of the polymer electrolyte membrane and the electrode substrate coincided. The stacked polymer electrolyte membrane and electrode substrate were then sandwiched between two gold-plated stainless steel block electrodes (the sandwiching surfaces were 3 cm squares), with the electrode substrate measuring 9 cm. 2Pressurization was applied to the area of ​​the 9cm² electrode so that the pressure was 4 MPa. During this process, the sides of the stainless steel block electrode were aligned parallel to the sides of the electrode substrate, and the centers of the stainless steel block electrode and the electrode substrate were aligned. A 2V DC voltage was applied between the gold-plated stainless steel block electrodes using a digital multimeter (KEITHLEY Model 196 SYSTEM DMM), and the current between the electrodes was measured. The obtained value was defined as the short-circuit current. This measurement was repeated 10 times, and the average value was calculated for the 9cm² area where pressure was applied to the electrodes. 2 The short-circuit current density was obtained by dividing by [a certain factor].

[0050] <Power generation performance of polymer electrolyte fuel cells> A catalyst solution was prepared by sequentially adding 1.00 g of platinum-supported carbon (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., platinum load: 50% by mass), 1.00 g of purified water, 8.00 g of “Nafion”® solution (5.0% by mass of “Nafion”® manufactured by Aldrich), and 18.00 g of isopropyl alcohol (manufactured by Nacalai Tesque).

[0051] Next, a 5cm x 5cm piece of "NAFRONT" (registered trademark) PTFE tape "TOMBO" (registered trademark) 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 fabricated. Next, a solid polymer electrolyte membrane "Nafion" (registered trademark) NRE-211CS (manufactured by DuPont), cut to 8cm x 8cm, was sandwiched between two PTFE sheets with catalyst layers. The sheets were then pressed in a flat plate press at 5MPa and 130°C for 5 minutes to transfer the catalyst layer to the solid polymer electrolyte membrane. After pressing, the PTFE sheets were peeled off to fabricate a solid polymer electrolyte membrane with a catalyst layer.

[0052] Next, a solid polymer electrolyte membrane with a catalyst layer was sandwiched between two electrode substrates, each cut to 5 cm x 5 cm, that had been prepared in each example and comparative example. The membrane electrode assembly was then pressed in a flat plate press at 3 MPa and 130°C for 5 minutes. The obtained membrane electrode assembly was incorporated into a single cell for fuel cell evaluation, and the voltage was measured when the current density was varied. A single-channel serpentine separator with a groove width, groove depth, and rib width of 1.0 mm was used as the separator. Unpressurized hydrogen was supplied to the anode side, and unpressurized air to the cathode side for evaluation.

[0053] To confirm the drainage properties of the electrode substrate, both hydrogen and air were humidified using a humidifier pot set to 70°C. The humidity at this time was 100%. The utilization rates of hydrogen and oxygen in the air were set to 70 mol% and 40 mol%, respectively, and the cell temperature was set to 70°C. Current density: 1.5 A / cm² 2 The output voltage was measured and used as an indicator of drainage performance.

[0054] (Example 1) Polyacrylonitrile carbon fibers manufactured by Toray Industries, Inc. were cut to a length of 7 mm, dispersed in water, and used in papermaking. They were then immersed in a 10% by weight aqueous dispersion of polyvinyl alcohol and dried, resulting in a carbon fiber basis weight of approximately 20 g / m². 2 A long sheet of carbon fiber was obtained and wound into a roll.

[0055] A dispersion (liquid resin composition) was prepared by mixing an aqueous dispersion of PTFE resin ("Polyflon" (registered trademark) PTFE Dispersion D-210C (manufactured by Daikin Industries, Ltd.)), acetylene black "Denka Black" (registered trademark) (manufactured by Denka Co., Ltd.), "TRITON" (registered trademark) X-100 (manufactured by Nakalai Tesque Co., Ltd.) as a surfactant, and water in a ratio of PTFE resin:acetylene black:surfactant:water = 6:11:1:82 (by weight). Carbon fiber sheets cut to 10cm x 10cm were immersed in the dispersion, and after being removed, excess liquid was squeezed with a mangle, and then dried at a temperature of 100°C for 10 minutes.

[0056] Next, the carbon fiber sheet, which had been immersed in the dispersion, was subjected to a heat-pressure treatment at 180°C for 5 minutes while being pressed with a flat plate press. During the heat-pressure treatment, spacers were placed in the flat plate press to adjust the distance between the upper and lower press plates so that the thickness after the heat-pressure treatment was 150 μm. Then, the electrode substrate was obtained by further heat treatment at 380°C for 10 minutes.

[0057] (Example 2) An electrode substrate was obtained in the same manner as in Example 1, except that a carbon fiber sheet was immersed in a liquid resin composition mixed in the ratio PTFE resin:acetylene black:surfactant:water = 8:15:1:76 (by weight).

[0058] (Example 3) During the heat and pressure treatment, the spacers of the flat plate press were changed to adjust the distance between the upper and lower press plates so that the thickness after treatment was 85 μm, and the density of the carbon fiber sheet after heat treatment was 0.60 g / cm³. 3 An electrode substrate was obtained in the same manner as in Example 1, except that it was configured as follows.

[0059] (Comparative Example 1) Acetylene black, graphite (manufactured by Wako Pure Chemical Industries, Ltd., flaky graphite, average particle size 15 μm), VGCF (manufactured by Showa Denko, fiber diameter 0.15 μm, fiber length 15 μm), surfactant, and water were mixed, and then an aqueous dispersion of PTFE was added and stirred. The weight ratio of PTFE resin:graphite:acetylene black:VGCF:surfactant:water was 4:21:13:4:1:57.

[0060] The resulting mixture was rolled into a sheet using a rolling machine, and the sheet-like mixture was heat-treated at 380°C for 10 minutes.

[0061] The heat-treated compound was then rolled again using a stretching roll machine to adjust its thickness and obtain an electrode substrate.

[0062] (Comparative Example 2) An electrode substrate was obtained in the same manner as in Example 1, except that a liquid resin composition was applied to a carbon fiber sheet using a die coater.

[0063] (Comparative Example 3) An electrode substrate was obtained in the same manner as in Example 1, except that a carbon fiber sheet was immersed in a liquid resin composition mixed in the ratio PTFE resin:acetylene black:surfactant:water = 9:9:1:81 (by weight).

[0064] (Example 4) An electrode substrate was obtained in the same manner as in Example 1, except that pressure was not applied using a flat plate press.

[0065] Table 1 shows the configuration of the electrode substrate and the various evaluation results for each example and comparative example.

[0066] [Table 1] [Industrial applicability]

[0067] The gas diffusion electrode substrate using the electrode substrate of the present invention can be suitably used as a gas diffusion electrode in fuel cells, particularly polymer electrolyte fuel cells. Furthermore, because the gas diffusion electrode substrate using the electrode substrate of the present invention can achieve a balance of conductivity, drainage, gas diffusion, and springiness, it is possible to manufacture polymer electrolyte fuel cells that exhibit high power generation performance even in operating environments where the fuel gas supply is large and a large amount of water is generated, such as high humidity, low temperature, and high current density conditions.

Claims

1. An electrode substrate comprising a carbon fiber sheet with fluororesin and carbon fine powder filled in the voids, The minimum value (F / C) among the average value on one surface (F / C-1), the average value on the center plane in the thickness direction (F / C-2), and the average value on the other surface (F / C-3) of the fluorine / carbon ratio (F / C) measured by scanning electron microscopy-energy dispersive X-ray spectroscopy. ｍｉｎ ) Maximum value (F / C ｍａｘ The ratio to ) is 0.80 or higher, Furthermore, the electrode substrate has at least a first peak located in the range of pore diameters of 0.5 μm or more and less than 120 μm, and a second peak located in the range of 10 nm or more and less than 0.5 μm, as measured by the mercury intrusion method, and the ratio (V1 / V2) of the peak volume of the first peak (V1) to the peak volume of the second peak (V2) is 1.0 to 7.

0.

2. Density is 0.50 g / cm³ ３ The electrode substrate according to claim 1, which is as follows:

3. The electrode substrate according to claim 1 or 2, wherein the carbon fiber sheet is a carbon fiber papermaking body.

4. The electrode substrate according to any one of claims 1 to 3, wherein the carbon fine powder content is 40% by weight or more and 70% by weight or less.

5. The electrode substrate according to any one of claims 1 to 4, wherein the fluororesin content is 10% by weight or more.

6. The electrode substrate according to any one of claims 1 to 5, wherein the carbon fine powder is contained in an amount of 200 parts by weight or more and 400 parts by weight or less per 100 parts by weight of the fluororesin.

7. The electrode substrate according to any one of claims 1 to 6, wherein it does not contain a thermosetting resin as a binder for binding the carbon fibers constituting the carbon fiber sheet.

8. The electrode substrate according to any one of claims 1 to 7, wherein the unpressurized thickness after burning treatment in an atmospheric atmosphere at 500°C for 1 hour is 1.5 times or more the unpressurized thickness before burning treatment.

9. The electrode substrate according to any one of claims 1 to 8, wherein the component filling the voids in the carbon fiber sheet does not include a thermosetting resin.

10. The short-circuit current density of the carbon fiber sheet is 80 mA / cm². ２ The electrode substrate according to any one of claims 1 to 9, which is as follows:

11. A carbon fiber sheet is immersed in a dispersion containing fluororesin and carbon fine powder in an immersion step, A heat treatment step to melt the fluororesin contained in the dispersion that was immersed in the immersion step, A method for manufacturing an electrode substrate according to claim 1, comprising having the characteristics described above.

12. A method for producing an electrode substrate according to claim 11, wherein the content of carbon fine powder is 40% by weight or more and 70% by weight or less of the total weight of the electrode substrate.

13. A method for manufacturing an electrode substrate according to claim 11 or 12, wherein the electrode substrate contains 200 to 400 parts by weight of carbon fine powder per 100 parts by weight of fluororesin.

14. A method for manufacturing an electrode substrate according to any one of claims 11 to 13, comprising a molding step of heating and pressurizing at a temperature of 100°C or higher after the immersion step and before the heat treatment step.

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