JP2019125457A - Fuel cell gas diffusion layer - Google Patents

Abstract

To provide a fuel cell gas diffusion layer improved in drainage while maintaining gas diffusion properties.SOLUTION: A fuel cell gas diffusion layer includes: a gas diffusion base material comprising a conductive porous body; and a porous and conductive water repellent layer formed on the gas diffusion base material. In the water repellent layer, a ratio D/Dof a maximum value Dof volumes of pores having diameters in the range of 0.01-0.5 μm to a volume Dof pores having a diameter of 1 μm in a pore distribution measured using a mercury porosimeter is 0.27 or more.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a gas diffusion layer for a fuel cell and a fuel cell provided with the gas diffusion layer, and more particularly, to a gas diffusion layer for a fuel cell having improved gas diffusivity and drainage performance, and the gas diffusion layer The present invention relates to a fuel cell provided.

  The fuel cell supplies fuel gas (hydrogen gas) and oxidant gas (oxygen gas) to the two electrodes connected electrically, and electrochemically oxidizes the fuel, thereby directly converting chemical energy into electricity. Convert to energy. The fuel cell is usually configured by laminating a plurality of unit cells each having a membrane electrode assembly in which an electrolyte membrane is sandwiched between a pair of electrodes as a basic structure. Above all, a solid polymer electrolyte fuel cell using a solid polymer electrolyte membrane as an electrolyte membrane has advantages such as easy size reduction and operation at low temperature, so that it is particularly portable and portable. It has attracted attention as a body power source.

In the solid polymer electrolyte fuel cell, the reaction of the following formula (1) proceeds at the anode (fuel electrode) supplied with hydrogen.
_{^{H 2 → 2H + + 2e -}} ··· (1)

  The electrons generated in the above equation (1) pass through an external circuit, work with an external load, and then reach the cathode (oxidizer electrode). On the other hand, the proton generated in the above formula (1) moves in the solid polymer electrolyte membrane from the anode side to the cathode side by electroosmosis in a state of being hydrated with water.

On the other hand, the reaction of the following formula (2) proceeds at the cathode.
_{^{2H + + 1 / 2O 2 +}} 2e - → H 2 O ··· (2)

Therefore, the chemical reaction shown in the following (3) progresses in the whole battery, an electromotive force is generated, and an electrical work is performed on the external load.
H ₂ + 1 / 2O ₂ → H ₂ O (3)

  Each of the anode and cathode electrodes generally has a structure in which a catalyst layer and a gas diffusion layer are laminated in order from the electrolyte membrane side. The catalyst layer usually contains an electrode catalyst such as platinum or a platinum alloy for promoting the above electrode reaction, a polymer electrolyte for securing proton conductivity, and a conductive material for securing electron conductivity. ing. In addition, the gas diffusion layer is usually formed using a conductive porous body that enables supply of reaction gas to the catalyst layer, discharge of excess water in the electrode, and the like.

  In a fuel cell provided with a solid polymer electrolyte membrane represented by a perfluorocarbon sulfonic acid resin membrane, it is important to maintain the wet state of the electrolyte membrane and the catalyst layer in order to secure the ion conductivity. Therefore, generally, the reaction gas is supplied to the electrode in a humidified state in advance.

  On the other hand, in the fuel cell, water is generated along with the power generation as described above. Most of the generated water is discharged to the outside of the cell together with unreacted gas discharged from the electrode (reaction gas not contributing to the electrode reaction) and humidified water supplied as steam. However, depending on the environment in the fuel cell, the product water and a part of the humidified water supplied as steam to the electrode together with the reaction gas condenses and exists in the liquid state in the electrode. At this time, if the condensed water (liquid water) is retained in the electrode, the supply of the reaction gas is hindered and the power generation performance is lowered. In particular, when the fuel cell is operated under highly humidified conditions, a large amount of liquid water is generated, and therefore it is necessary to secure the discharge performance of the liquid water of the electrode.

  Therefore, it is desirable to rapidly discharge the liquid water in the electrode to the outside of the cell to prevent retention in the electrode.

  Therefore, many techniques have been proposed to improve the drainage performance of the electrode of the fuel cell. For example, Patent Document 1 describes a fuel cell gas diffusion layer having a conductive water repellent layer, wherein the conductive water repellent layer has a first peak in the range of 1 μm to 10 μm in the through pore distribution. And a second gas diffusion layer having a second peak in the range of 0.05 .mu.m to 0.5 .mu.m.

JP, 2010-129310, A

  As in the gas diffusion layer described in Patent Document 1, it is presumed that providing a through pore has a certain effect, but if the volume of the through pore having a size suitable for drainage is not sufficient, liquid water That is, there is a problem that so-called flooding occurs and performance does not increase. In addition, even if there is a sufficient volume of through-pores of a size suitable for drainage, it is easy to dry the electrolyte membrane and the catalyst layer under the condition that drainage is not required, for example, at high temperature performance, only by improving drainage. There is a problem that the performance is degraded.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide a fuel cell gas diffusion layer having improved drainage performance while maintaining gas diffusibility.

According to the present invention to solve the above problems,
[1] A fuel cell gas diffusion layer comprising: a gas diffusion base composed of a conductive porous body; and a water-repellent layer which is formed on the gas diffusion base and is porous and conductive. The water repellent layer has a maximum pore volume D _max in the range of pore diameters 0.01 to 0.5 μm and a pore volume D at a pore diameter 1 μm in the pore distribution measured using a mercury porosimeter. _A fuel cell gas diffusion layer having a ratio of ₁ and D ₁ / D _max of 0.27 or more is provided.

Further, the present invention is characterized by the following items.
[2]
In the above-mentioned [1], D ₁ / D _max is 2.0 or less.
[3]
In the above [1] or [2], the water repellent layer is formed of a mixture of carbon materials having different shapes and / or sizes, or a mixture of fibrous carbon materials.
[4]
In any of the above [1] to [3], the water repellent layer is formed of a mixture of spherical carbon black and scaly graphite.
[5]
In any of the above [1] to [3], the water repellent layer is formed of a mixture of spherical carbon black and thermal black having a particle diameter larger than that of the carbon black.
[6]
In any of the above [1] to [3], the water repellent layer is formed of a fibrous carbon material.

Further according to the invention,
[7] A fuel cell provided with the above fuel cell gas diffusion layer is provided.

In the fuel cell gas diffusion layer of the present invention, by making D ₁ / D _max in the water repellent layer 0.27 or more, the drainage property is improved while maintaining the gas diffusibility, and excessive liquid water is obtained. It is possible to prevent drying as well as discharge, and to improve the output point of the fuel cell.

It is sectional drawing which shows the structure of the single cell provided with the gas diffusion layer for fuel cells of this invention. It is a graph which shows 70 degreeC output point performance of the fuel cell produced in the Example and the comparative example. It is a graph which shows the 100 degreeC output point performance of the fuel cell produced in the Example and the comparative example.

<Structure of Fuel Cell>
Hereinafter, the present invention will be described using the drawings. However, these drawings and the description thereof are illustrative and do not limit the present invention.

  FIG. 1 is a cross-sectional view showing the configuration of a single cell provided with a fuel cell gas diffusion layer according to the present invention. In FIG. 1, a single cell 1 has a structure in which a membrane electrode assembly 5 is sandwiched between two separators 6 and 7. The membrane electrode assembly 5 includes an electrolyte membrane 2, a cathode (oxidizer electrode) 3 provided on one surface of the electrolyte membrane 2, and an anode (fuel electrode) 4 provided on the other surface. . The cathode 3 has a multilayer structure in which a cathode catalyst layer 31 and a gas diffusion layer 32 are laminated in order from the electrolyte membrane 2 side. The anode 4 also has a multilayer structure in which an anode catalyst layer 41 and a gas diffusion layer 42 are laminated in order from the electrolyte membrane side. The gas diffusion layer 32 includes a water repellent layer 33 and a gas diffusion substrate 34, and the gas diffusion layer 42 also includes a water repellent layer 43 and a gas diffusion substrate 44. In the separators 6 and 7, an oxidant gas flow channel 6a and a fuel gas flow channel 7a are provided, respectively.

  In the unit cell 1, the cathode 3 and the anode 4 are disposed on both sides of the solid polymer electrolyte membrane 2, and generally joined together by hot pressing to form a membrane electrode assembly 5, and then a membrane electrode assembly The cathode separator 6 and the anode separator 7 are disposed on both sides of the electrode 5. In the unit cell 1, a fuel gas containing hydrogen is supplied to the anode 4 side, and an oxidant gas such as air containing oxygen is supplied to the cathode 3 side. Thereby, as described above, an electrochemical reaction occurs at the contact surface between the solid polymer electrolyte membrane 2 and the cathode catalyst layer 31 and the anode catalyst layer 41.

<Gas diffusion layer>
The gas diffusion layers 32 and 42 according to the present invention have a configuration in which the water repellent layers 33 and 43 are provided on the gas diffusion substrates 34 and 44, and the water repellent layer 33 is in contact with the cathode catalyst layer 31. The gas diffusion substrate 34 is in contact with the cathode separator 6. Similarly, the water repellent layer 43 is in contact with the anode catalyst layer 41, and the gas diffusion base 44 is in contact with the anode separator 7.

  The gas diffusion substrate 34 is made of a conductive porous material, disperses the oxidant gas supplied through the oxidant gas flow channel 6a, uniformly supplies the oxidant gas to the cathode catalyst layer 31, and It has a role of discharging the water produced by the oxidation reaction in the cathode catalyst layer 31 to the outside of the single cell. In addition, the gas diffusion substrate 44 disperses the fuel gas supplied through the fuel gas flow path 7a, uniformly supplies the fuel gas to the anode catalyst layer 41, and further, excess water in the anode catalyst layer 41 is It has a role of discharging to the outside of a single cell.

  Therefore, both of the gas diffusion substrates 34 and 44 are generally used as a diffusion layer in a fuel cell, and can be used as a material for forming a gas diffusivity, conductivity, and gas diffusion layer that can efficiently supply a gas to the catalyst layer. Those having the required strength, for example, carbon porous materials such as carbon paper, carbon cloth and carbon felt, titanium, aluminum, copper, nickel, nickel-chromium alloy, copper and its alloy, silver, aluminum alloy, A conductive porous body such as a metal mesh or a metal porous body made of a metal such as zinc alloy, lead alloy, titanium, niobium, tantalum, iron, stainless steel, gold, platinum or the like can be used.

  The water repellent layers 33, 43 provided on the gas diffusion substrates 34, 44 are porous and conductive. The water repellent layer 33 has a function of discharging the generated water produced in the cathode catalyst layer 31 to the gas diffusion substrate 34 while circulating the oxidant gas dispersed through the gas diffusion substrate 34. Therefore, the water repellent layer 33 needs to have both the water draining property and the gas permeability. Further, the water repellent layer 43 has a role of discharging excess water in the anode catalyst layer 41 to the gas diffusion substrate 44 while circulating the fuel gas dispersed through the gas diffusion substrate 44. Therefore, the water repellent layer 43 needs to have both gas permeability and water dischargeability.

In the present invention, the water repellent layer 33, 43 has a ratio of the maximum value D _max of the pore volume in the range of 0.01 to 0.5 μm of pore diameter and the pore volume D _{1 at 1} μm of pore diameter. , D ₁ / D _max is 0.27 or more. Here, a pore with a pore diameter of 1 μm is a pore through which liquid water passes and a pore in the range of a pore diameter of 0.01 to 0.5 μm is a pore through which a gas passes, so D ₁ / D _max is 0 By setting it as 27 or more, drainage will be improved while maintaining gas diffusivity. Also, D ₁ / D _max may be, for example, 0.3 or more, 0.40 or more, or 0.50 or more, and is 2.0 or less, 1.5 or less, or 1.0 or less. In addition, the said pore distribution is the value calculated | required using the mercury porosimeter by the mercury intrusion method generally used in the said field | area.

In order to adjust this D ₁ / D _max , a mixture of carbon materials having different shapes and / or sizes, such as spherical, flaky, fibrous, etc., as materials constituting the water repellent layers 33 and 43, or It is formed of a mixture of fibrous carbon materials. Such materials include carbon blacks such as furnace black, acetylene black, lamp black and thermal black, flaky graphite, flaky graphite, earthy graphite, artificial graphite, graphite such as expanded graphite, carbon nanotubes, carbon nanofibers, Examples thereof include linear carbon such as vapor grown carbon fiber (VGCF) and carbon fiber milled fiber.

  Here, the particle diameter of the spherical material is preferably 100 nm to 1 μm. The length of the flaky material is preferably 1 to 10 μm, and the aspect ratio is preferably 1.2 to 40. And it is preferable that the length of fibrous material is 10-20 micrometers.

Then, the water repellent layer is formed using a mixture of spherical materials different in particle diameter, using a mixture of spherical material and flaky or fibrous material, or using a fibrous material, in which case The volume of the pores in the water repellent layer can be adjusted to adjust D ₁ / D _max by adjusting the quality of the material, particle diameter, length, aspect ratio, etc. More specifically, a water repellent layer may be formed from a mixture of spherical carbon black and flaky graphite, or a mixture of spherical carbon black and a thermal black having a larger particle size than the carbon black, or a carbon nanotube. preferable.

Other Components of Fuel Cell
In the fuel cell configured by using the gas diffusion layer of the present invention, as the electrolyte membrane 2, the cathode catalyst layer 31, the anode catalyst layer 41, and the separator 6, those generally used in fuel cells can be used. .

  As the separator, for example, a carbon separator containing a carbon fiber at a high concentration and made of a composite material with a resin, a metal separator using a metal material, or the like can be used. Examples of the metal separator include those made of a metal material excellent in corrosion resistance, and those coated with a coating that enhances the corrosion resistance by coating the surface with carbon, a metal material excellent in corrosion resistance, etc. . The separator usually has a hydrophilic surface to ensure drainage.

  Examples of the electrolyte membrane include fluorine-based polymer electrolytes such as perfluorocarbon sulfonic acid resin films represented by Nafion (trade name), and hydrocarbon-based polymers such as polyether ketone, polyether ether ketone, and polyether sulfone. Further, those obtained by forming a hydrocarbon-based polymer electrolyte or the like in which a proton-conductive group such as a sulfonic acid group, a phosphoric acid group, or a hydroxyl group is introduced into a film shape can be mentioned.

  Each catalyst layer is usually configured using at least an electrode catalyst, a conductive material, and a polymer electrolyte. Examples of the electrode catalyst include those generally used in fuel cells, which have catalytic activity for the oxidation reaction of hydrogen at the anode or the reduction reaction of oxygen at the cathode, for example, platinum, or Alloys of metals such as ruthenium, iron, nickel, manganese and the like with platinum and the like can be exemplified.

  It is preferable that the electrode catalyst is usually supported by a conductive material. The conductive material is not particularly limited as long as it has conductivity, and includes metal particles, metal fibers, etc. in addition to conductive carbon materials such as carbon particles and carbon fibers.

  The polymer electrolyte is contained in the catalyst layer for the purpose of imparting proton conductivity to the catalyst layer, fixing the electrode catalyst and the conductive material, and the like. Examples of the polymer electrolyte include those having proton conductivity and generally used to constitute an electrolyte membrane of a fuel cell, and examples thereof include perfluorocarbons such as Nafion (trade name, manufactured by DuPont) In addition to fluorine-based polymer electrolytes represented by sulfonic acid resins, hydrocarbon-based polymers such as polyetheretherketone, polyetherketone, polyethersulfone, polyphenylene sulfide, polyphenylene ether, etc., sulfonic acid group, carboxylic acid group, phosphorus Examples thereof include hydrocarbon-based electrolytes in which a proton conductive group such as an acid group or a boronic acid group is introduced.

  The catalyst layer can be formed by using a catalyst ink in which the above-described materials are dissolved and / or dispersed in a solvent. The solvent may be appropriately selected depending on the electrode catalyst and the polymer electrolyte to be used. For example, alcohols such as methanol, ethanol, propanol and propylene glycol, N, N-dimethylformamide, N, N-diethylformamide, N, N-dimethylacetamide, N, N-diethylacetamide and the like, or mixtures thereof A mixture with water or water can be used, but is not limited thereto.

  Specifically, an electrolyte membrane-catalyst layer assembly or a catalyst layer-gas diffusion layer assembly can be obtained by applying a catalyst ink to the electrolyte membrane or the above-mentioned gas diffusion layer sheet and drying. Alternatively, a catalyst ink is applied to a transfer substrate to prepare a catalyst layer transfer sheet, and the electrolyte membrane / catalyst layer assembly or catalyst layer / gas diffusion layer is transferred by transferring the catalyst layer to the electrolyte membrane or gas diffusion layer sheet. A conjugate can be obtained. The application method and the drying method of the catalyst ink are not particularly limited.

  The electrolyte membrane-catalyst layer assembly or the catalyst layer-gas diffusion layer assembly obtained as described above has a gas diffusion layer sheet or electrolyte membrane so that the catalyst layer is positioned between the electrolyte membrane and the gas diffusion layer. The membrane electrode assembly can be obtained by bonding with As a bonding method, a thermocompression bonding etc. are mentioned, for example. The obtained membrane-electrode assembly is further sandwiched by a separator to obtain a single cell.

  As shown in Table 1 below, carbon type and amount, and polytetrafluoroethylene (PTFE) were weighed, put into 400 parts by weight of water per 100 parts by weight of carbon, and stirred to obtain a microporous layer (MPL ) Made a paste. At this time, 10 parts by weight of a dispersing material (nonionic surfactant) was added for the purpose of improving the dispersion. Next, the obtained MPL paste is applied onto a 150 μm thick diffusion layer base material (TGP-H-060, manufactured by Toray Industries, Inc.), and the amount of coating is adjusted to a thickness of 20 μm. It was coated.

The carbon types used are as follows.
Carbon black: manufactured by Denki Kagaku Kogyo Co., Ltd., trade name "Denka Black"
Carbon A: Cancarb Company, trade name "thermal black"
Carbon B: Nippon Graphite Co., Ltd., trade name "exfoliated graphite powder"
Fibrous carbon: Showa Denko KK, trade name "VGCF-H"

  The pore distribution of the thus prepared gas diffusion layer was measured by mercury porosimetry using mercury porosimetry and evaluated. Moreover, the electrical resistance was calculated | required per unit area, and the ratio with respect to the comparative example 1 was calculated. The obtained results are shown in Table 2 below. In addition, although the pore distribution measured by said mercury intrusion method was measured in the state which provided the water-repellent layer on the gas diffusion base material, the result shown in the following Table 2 is a pore of a diffusion layer base material It does not contain and shows pore distribution of a water repellent layer.

  The obtained gas diffusion layers are disposed on both sides of the membrane electrode assembly, and a separator is combined to constitute a fuel cell, the cell temperature is 40 ° C., pure hydrogen for the anode and air for the cathode are each 500 ml / ml. The flow rate was supplied via a humidifier at a flow rate of 1000 ml / min. As this membrane electrode assembly, Nafion (registered trademark) is used as an electrolyte membrane, and a catalyst that promotes an electrochemical reaction, such as a porous body provided with platinum or an alloy of platinum and another metal, is used as an electrode catalyst layer. The membrane electrode assembly generally used in a fuel cell was used, which had gas permeability. The measurement results of the output voltage at 70 ° C. and 100 ° C. of the fuel cell using each gas diffusion layer of the example and the comparative example are shown in the following Table 3 and FIG. 2 and FIG.

As shown in Table 3 and FIG. 2, the 70 ° C. output point performance is improved at D ₁ / D _max of 0.27 or more, and it is presumed that 1 μm pores function well for drainage . Also, as shown in Table 3 and FIG. 3, improving the diffusivity generally tends to lower the high temperature performance, but there is no contradiction. This is presumed to be due to the decrease in the electronic resistance and the thermal resistance.

DESCRIPTION OF SYMBOLS 1 single cell 2 solid polymer electrolyte membrane 3 cathode 4 anode 5 membrane electrode assembly 6 cathode separator 7 anode separator 31 cathode catalyst layer 41 anode catalyst layer 32 cathode gas diffusion layer 42 anode gas diffusion layer 33 water repellent layer 43 water repellent layer 34 Gas Diffusion Base 44 Gas Diffusion Base

Claims (7)

A gas diffusion substrate composed of a conductive porous body,
A fuel cell gas diffusion layer comprising a porous and conductive water repellent layer formed on the gas diffusion substrate, comprising:
The water repellent layer has a maximum pore volume D _max in the range of pore diameters 0.01 to 0.5 μm and a pore volume D at a pore diameter 1 μm in the pore distribution measured using a mercury porosimeter. _A fuel cell gas diffusion layer having a ratio of ₁ and D ₁ / D _max of 0.27 or more. The fuel cell gas diffusion layer according to claim 1, wherein the D ₁ / D _max is 2.0 or less.   3. The fuel cell gas diffusion layer according to claim 1, wherein the water repellent layer is formed of a mixture of carbon materials different in shape and / or size or a mixture of fibrous carbon materials.   The fuel cell gas diffusion layer according to any one of claims 1 to 3, wherein the water repellent layer is formed of a mixture of spherical carbon black and scaly graphite.   The fuel cell gas diffusion layer according to any one of claims 1 to 3, wherein the water repellent layer is formed of a mixture of spherical carbon black and thermal black having a larger particle size than the carbon black. .   The fuel cell gas diffusion layer according to any one of claims 1 to 3, wherein the water repellent layer is formed of a fibrous carbon material.   A fuel cell comprising the fuel cell gas diffusion layer according to any one of claims 1 to 6.

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