阅读说明：本技术 燃料电池用气体扩散层、膜电极接合体以及燃料电池 (Gas diffusion layer for fuel cell, membrane electrode assembly, and fuel cell ) 是由 川岛勉 田口良文 江崎贤一 于 2019-03-19 设计创作，主要内容包括：本公开提供一种燃料电池用气体扩散层、膜电极接合体以及燃料电池,使用如下燃料电池用气体扩散层,该燃料电池用气体扩散层由以导电性粒子、导电性纤维以及高分子树脂为主要成分的多孔质构件构成,在所述多孔质构件的内部形成有所述导电性纤维的凝集体,所述多孔质构件的任意的截面中的所述凝集体的面积比率为0.5％以上且8％以下。此外,使用具备上述燃料电池用气体扩散层的膜电极接合体。进而,使用具备上述燃料电池用气体扩散层的燃料电池。(Disclosed is a gas diffusion layer for a fuel cell, which is composed of a porous member that contains conductive particles, conductive fibers, and a polymer resin as the main components, wherein aggregates of the conductive fibers are formed inside the porous member, and wherein the area ratio of the aggregates in any cross section of the porous member is 0.5% to 8%. Further, a membrane electrode assembly including the fuel cell gas diffusion layer is used. Further, a fuel cell including the gas diffusion layer for a fuel cell is used.)

1. A gas diffusion layer for a fuel cell,

Comprising a porous member mainly composed of conductive particles, conductive fibers and a polymer resin,

An aggregate of the conductive fibers is formed inside the porous member,

The area ratio of the aggregates in any cross section of the porous member is 0.5% to 8%.

2. the gas diffusion layer for a fuel cell according to claim 1,

The amount of the conductive fibers in the porous member is larger than the amount of the conductive particles.

3. The gas diffusion layer for a fuel cell according to claim 1,

The ratio of the conductive particles present in the aggregate is 10% or less.

4. The gas diffusion layer for a fuel cell according to claim 1,

The diameter of the aggregate is 1-20 μm.

5. The gas diffusion layer for a fuel cell according to claim 1,

The conductive fiber is a carbon nanotube having a fiber diameter of 50nm to 300nm and a fiber length of 0.5 μm to 50 μm.

6. the gas diffusion layer for a fuel cell according to claim 1,

The porous member includes 5 wt% or more and less than 35 wt% of the conductive particles, includes 35 wt% or more and 80 wt% or less of the conductive fibers, and includes 10 wt% or more and 40 wt% or less of the polymer resin.

7. The gas diffusion layer for a fuel cell according to claim 1,

The polymer resin is polytetrafluoroethylene.

8. The gas diffusion layer for a fuel cell according to claim 7,

The polymer resin contains fibrous polytetrafluoroethylene and particulate polytetrafluoroethylene.

9. The gas diffusion layer for a fuel cell according to claim 8,

the particle-shaped polytetrafluoroethylene has a diameter of 0.1 to 10 [ mu ] m.

10. The gas diffusion layer for a fuel cell according to claim 8,

The area ratio of the particulate polytetrafluoroethylene in the arbitrary cross section is 0.1% or more and 1% or less.

11. The gas diffusion layer for a fuel cell according to claim 1,

The porous member has a cumulative pore volume of 1.0mL/g to 1.7mL/g, and a peak of pore diameter distribution of 0.05 μm to 0.5 μm.

12. The gas diffusion layer for a fuel cell according to claim 1,

The porosity of the porous member is 65% or more and 75% or less.

13. The gas diffusion layer for a fuel cell according to claim 1,

the porous member was pressed at a surface pressure of 7kgf/cm²The thickness after compression is 85% to 98% of the thickness before compression.

14. The gas diffusion layer for a fuel cell according to claim 1,

the gurley number of the porous member is 5sec/100mL or more and 150sec/100mL or less.

15. The gas diffusion layer for a fuel cell according to claim 1,

The porous member has a tensile breaking strength of 0.05N/mm²The above is a self-supporting film supported only by the conductive particles, the conductive fibers, and the polymer resin.

16. The gas diffusion layer for a fuel cell according to claim 1,

The thickness of the porous member is 70 to 200 [ mu ] m.

17. A membrane-electrode assembly comprising the gas diffusion layer for a fuel cell according to claim 1.

18. A fuel cell comprising the gas diffusion layer for a fuel cell according to claim 1.

Technical Field

The present disclosure relates to a gas diffusion layer provided in a membrane electrode assembly used in a fuel cell.

Background

An example of the fuel cell is a polymer electrolyte fuel cell. The basic principle of the polymer electrolyte fuel cell is that one surface of a hydrogen ion conductive polymer electrolyte membrane is exposed to a fuel gas such as hydrogen, the other surface is exposed to oxygen, water is synthesized by a chemical reaction through the electrolyte membrane, and reaction energy generated at this time is extracted electrically.

A unit cell of a polymer electrolyte fuel cell includes a membrane electrode assembly (hereinafter, referred to as MEA) and a pair of conductive separators disposed on both surfaces of the MEA.

The MEA includes a hydrogen ion conductive polymer electrolyte membrane and a pair of electrode layers sandwiching the electrolyte membrane. The pair of electrode layers has: catalyst layers formed on both surfaces of the polymer electrolyte membrane and mainly composed of carbon powder supporting a platinum group catalyst; and a gas diffusion layer formed on the catalyst layer and having a current collecting function, gas permeability, and water repellency.

The gas diffusion layer serves to uniformly supply the gas supplied from the separator to the catalyst layer, and therefore needs to have good gas permeability and gas diffusion properties. In addition, the gas diffusion layer is required to have excellent conductivity as a conductive path for electrons between the catalyst layer and the separator. Therefore, the gas diffusion layer uses an electrically conductive porous member.

In addition, the gas diffusion layer is required to have high hydrophobicity so that excess water generated by the cell reaction in the catalyst layer is quickly removed and discharged to the outside of the MEA system, and the pores of the gas diffusion layer are not clogged with generated water. Therefore, a gas diffusion layer is generally used in which an electrically conductive porous member is subjected to a water repellent treatment with a fluororesin or the like, and a water repellent layer containing a carbon powder and a hydrophobic resin such as a fluororesin as main components is provided on the side of an electrically conductive substrate that is in contact with a catalyst layer.

By thus subjecting the conductive base material to the water-repellent treatment, clogging of the pores of the gas diffusion layer due to the generated water is prevented. Further, by making the water-repellent layer more hydrophobic than the electrically conductive base material, excess moisture generated in the catalyst layer can be quickly discharged out of the MEA system.

Such a gas diffusion layer is disclosed in, for example, patent documents 1, 2, and 3.

The gas diffusion layer of patent document 1 is composed of a porous member containing conductive particles and a polymer resin as main components, and carbon fibers having a weight smaller than that of the polymer resin are added.

The gas diffusion layer of patent document 2 is composed of a porous member containing conductive particles and a polymer resin as main components and carbon fibers less in weight than the polymer resin, and has pores of 0.01to 0.05 μm and pores of 1to 200 μm inside.

The gas diffusion layer of patent document 3 is a sheet having a thickness of 0.05 to 2mm, which is obtained by papermaking of a mixture of fine carbon fibers having a fiber diameter of 0.5 to 500nm and a fiber length of 1000 μm or less and having a hollow structure in the central axis, conductive particles and hydrophobic resin particles.

prior art documents

Patent document

Patent document 1: japanese patent No. 4938133

Patent document 2: international publication No. 2017/085901

Patent document 3: international publication No. 2005/043656

Disclosure of Invention

Problems to be solved by the invention

However, it is desired to further improve the gas permeability and water discharge of the gas diffusion layer.

The purpose of the present disclosure is to provide a gas diffusion layer for a fuel cell, a membrane electrode assembly, and a fuel cell, which have sufficient gas permeability and water drainage properties.

Means for solving the problems

As a result of intensive studies to achieve the above object, the inventors of the present disclosure have found that a gas diffusion layer is composed of conductive particles, conductive fibers and a polymer resin, and the presence of aggregates of the conductive fibers in the gas diffusion layer can significantly improve gas permeability and water discharge, which are problems of the gas diffusion layer, and improve power generation performance of a fuel cell.

To achieve the object, the present disclosure is configured as follows. That is, the gas diffusion layer for a fuel cell according to the present disclosure is composed of a porous member containing conductive particles, conductive fibers, and a polymer resin as main components, aggregates of the conductive fibers are formed inside the porous member, and the area ratio of the aggregates in any cross section of the porous member is 0.5% or more and 8% or less.

Effects of the invention

The present disclosure can provide a gas diffusion layer for a fuel cell, a membrane electrode assembly, and a fuel cell having sufficient gas permeability and water discharge properties.

Drawings

Fig. 1 is a schematic view of a polymer electrolyte fuel cell stack according to an embodiment of the present disclosure.

fig. 2 is a cross-sectional view showing a schematic structure of a polymer electrolyte fuel cell according to an embodiment of the present disclosure.

Fig. 3A is a schematic diagram showing a schematic structure of a cross section of a gas diffusion layer according to an embodiment of the present disclosure.

Fig. 3B is an enlarged schematic view of a cross section of a gas diffusion layer of an embodiment of the present disclosure.

Fig. 4A is a cross-sectional SEM photograph of a gas diffusion layer of an embodiment of the present disclosure.

Fig. 4B is a cross-sectional SEM photograph of the gas diffusion layer of an embodiment of the present disclosure.

Fig. 5 is a flowchart for explaining a method of manufacturing a gas diffusion layer according to an embodiment of the present disclosure.

Fig. 6A is a schematic diagram showing a schematic structure of a cross section of a modified example of the gas diffusion layer according to the embodiment of the present disclosure.

Fig. 6B is an enlarged schematic view of a cross section of a modification of the gas diffusion layer of the embodiment of the present disclosure.

Fig. 7 is a cross-sectional SEM photograph of a modification of the gas diffusion layer according to the embodiment of the present disclosure.

Description of the symbols:

100: fuel cell, 1: polymer electrolyte membrane, 2: catalyst layer, 2 a: anode catalyst layer, 2 b: cathode catalyst layer, 3: gas diffusion layer, 3M: gas diffusion layer, 3 a: anode-side gas diffusion layer, 3 b: cathode-side gas diffusion layer, 4: spacer, 4 a: anode-side spacer, 4 b: cathode-side spacer, 5: fluid flow path, 6: rib, 10: battery cell, 11: a collector plate, 12: insulating plate, 13: end plate, 30: porous member, 31: conductive particles, 32: conductive fiber, 33F: fibrous polymer resin, 33P: particulate polymer resin, 34: and (4) aggregating.

Detailed Description

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

A basic configuration of a fuel cell 100 according to an embodiment of the present disclosure will be described with reference to fig. 1. Fig. 1 is a schematic view of a polymer electrolyte fuel cell stack according to the present embodiment. The present embodiment is not limited to the polymer electrolyte fuel cell, and can be applied to various fuel cells.

< Fuel cell 100>

As shown in fig. 1, a fuel cell 100 is formed by stacking a plurality of basic unit cells 10, and compressing and fastening them with a predetermined load using current collecting plates 11, insulating plates 12, and end plates 13 disposed on both sides of the stacked cells 10.

The current collector plate 11 is a gas impermeable conductive material. For example, copper, brass, or the like is used for collector plate 11. Current collecting plate 11 is provided with a current collecting terminal (not shown) from which a current is collected during power generation.

The insulating plate 12 is made of an insulating material such as resin. The insulating plate 12 is made of, for example, a fluorine resin or a PPS resin.

The end plate 13 fastens and holds the plurality of stacked battery cells 10, the current collecting plate 11, and the insulating plate 12 with a predetermined load by a pressing means not shown. The end plate 13 is made of a highly rigid metal material such as steel.

Fig. 2 is a sectional view showing a schematic structure of the battery cell 10. The cell 10 has a structure in which the MEA20 is sandwiched between the anode-side separator 4a and the cathode-side separator 4 b.

Hereinafter, the anode-side separator 4a and the cathode-side separator 4b are collectively referred to as the separators 4. The same description applies to other components when a plurality of components are collectively described.

The spacer 4 is formed with a fluid flow path 5. A fluid flow path 5 for a fuel gas is formed in the anode-side separator 4 a. A fluid flow path 5 for the oxidant gas is formed in the cathode-side separator 4 b. As the spacer 4, a carbon-based or metal-based material can be used.

The fluid flow path 5 is a groove formed in the spacer 4. A rib 6 is provided around the fluid flow path 5.

The MEA20 includes a polymer electrolyte membrane 1, a catalyst layer 2, and a gas diffusion layer 3. An anode catalyst layer 2a and a cathode catalyst layer 2b (collectively referred to as catalyst layers 2) are formed on both sides of a polymer electrolyte membrane 1 that selectively transports hydrogen ions, and an anode-side gas diffusion layer 3a and a cathode-side gas diffusion layer 3b (collectively referred to as gas diffusion layers 3) are disposed on the outer sides thereof.

The polymer electrolyte membrane 1 is not particularly limited as long as it has proton conductivity, for example, using perfluorocarbon sulfonic acid copolymer.

As the catalyst layer 2, a layer including a carbon material on which catalyst particles such as platinum are supported and a polymer electrolyte can be used.

< example of gas diffusion layer construction >

Next, the structure of the gas diffusion layer 3 according to the embodiment of the present disclosure will be described in detail with reference to fig. 3A and 3B.

Fig. 3A is a cross-sectional view showing a schematic structure of the gas diffusion layer 3. Fig. 3B is an enlarged cross-sectional view of the gas diffusion layer 3. The gas diffusion layer 3 is composed of a porous member 30 mainly composed of conductive particles 31, conductive fibers 32, and a polymer resin 33. In the porous member 30, the amount of the conductive fibers 32 is larger than the amount of the conductive particles 31. As a result, the porous member 30 has the aggregate 34 of the conductive fibers 32 therein.

As the conductive particles 31, carbon materials such as carbon black, graphite, and activated carbon can be used. In particular, carbon black having high conductivity and large pore volume is preferably used as the conductive particles 31. In addition, as the carbon black, acetylene black, ketjen black, furnace black, and Vulcan can be used for the conductive particles 31. In particular, acetylene black having a small impurity content or ketjen black having a large specific surface area and high conductivity is preferably used as the conductive particles 31.

For example, carbon nanotubes can be used as the conductive fibers 32. The carbon nanotubes as the conductive fibers 32 preferably have a fiber diameter of 50nm to 300nm, and a fiber length of 1 μm to 50 μm. The reason for this is as follows.

when the fiber diameter of the carbon nanotube is less than 50nm or the fiber length is less than 1 μm, the gas diffusion layer 3 cannot have a strength having only a structure (so-called self-supporting structure) indicated by conductive particles, conductive fibers, and a polymer resin.

On the other hand, when the fiber diameter of the carbon nanotube is larger than 300nm or the fiber length is longer than 50 μm, pores formed in the gaps between the conductive fibers 32 become large, and thus the function as the water retentivity of the gas diffusion layer is lowered. In particular, the cell performance of the fuel cell 100 during the low humidification operation is degraded. Accordingly, the values of the fiber diameter and the fiber length of the carbon nanotubes as the conductive fibers 32 are preferably set within the above ranges.

Examples of the material of the polymer resin 33 include PTFE (polytetrafluoroethylene), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PVDF (polyvinylidene fluoride), ETFE (tetrafluoroethylene-ethylene copolymer), PCTFE (polychlorotrifluoroethylene), PFA (polyvinyl fluoride-perfluoroalkyl vinyl ether copolymer), and the like. In particular, PTFE is preferably used as the material of the polymer resin 33 from the viewpoint of heat resistance, hydrophobicity, and chemical resistance. The raw material form of PTFE includes a dispersion, a powder, and the like, and the use of a dispersion is more preferable because it is excellent in dispersibility.

The polymer resin 33 functions as a binder for binding the conductive particles 31 to each other. The polymer resin 33 is hydrophobic. This prevents water from accumulating in the pores in the gas diffusion layer 3 and inhibiting gas permeation.

< aggregate 34 of conductive fibers 32 >

The aggregate 34 of the conductive fibers 32 is an aggregate of conductive fibers that is not broken in the dispersing step in the production of the conductive fibers 32 and remains as a solidified material. Only 10% or less of the conductive particles 31 are present in the aggregate 34 of the conductive fibers 32. The reason for this is as follows.

The aggregate 34 has a structure in which the conductive fibers 32 are intertwined with each other. The diameter (pore diameter) of the pores, which are the gaps between the conductive fibers 32 constituting the aggregate 34, is 0.05 μm or more and 0.5 μm or less. On the other hand, when the conductive particles 31 are made of, for example, carbon black such as acetylene black or ketjen black, the size of the 1 st aggregates (aggregates) in which several tens of regions (1 st particles) having a particle diameter of 0.01 μm to 0.05 μm are fused together is 0.1 μm to 1 μm. The aggregates form secondary aggregates (agglomerated masses) by physical forces such as van der waals forces, but the agglomerated masses are hardly dispersed to the size of the aggregates completely in the dispersing step because of their strong bonding strength. Therefore, only a small amount of the conductive particles 31 such as carbon black enters the pores of the aggregates 34 of the conductive fibers 32. Thus, as described above, only 10% or less of the conductive particles 31 are present in the aggregate 34 of the conductive fibers 32.

The aggregate 34 of the conductive fibers 32 has a short diameter and a long diameter both in the range of 1to 20 μm. This is because, when the size of the aggregate 34 is less than 1 μm, the aggregate cannot sufficiently function as a gas diffusion path or a water discharge path, and when the aggregate exceeds 20 μm, the conductive particles 31 hardly exist between the conductive fibers 32, and thus the conductivity is lowered.

In any cross section of the gas diffusion layer 3, the area ratio of the aggregates 34 is 0.5% or more and 5% or less. The reason for this is as follows. That is, when the area ratio is less than 0.5%, sufficient gas diffusibility and water drainage cannot be ensured, and the battery performance is degraded. On the other hand, if the area ratio is more than 5%, the conductivity decreases, and the battery performance decreases.

< method for calculating area ratio of aggregate 34>

the calculation method of the area ratio of the aggregates 34 of the conductive fibers 32 in an arbitrary cross section of the gas diffusion layer 3 (porous member 30) is as follows. First, the gas diffusion layer 3 is cut, and after polishing the cross section, a photograph of the cross section is taken by SEM or optical microscope. Then, by selecting 1 μm to 20 μm aggregates from the cross-sectional photograph, the area ratio can be calculated from the area ratio of the aggregates to the total cross-sectional area.

fig. 4A is a cross-sectional SEM photograph of the gas diffusion layer 3, and fig. 4B is an enlarged photograph of the cross-sectional SEM of the gas diffusion layer 3. In fig. 4A and 4B, the conductive particles 31, the conductive fibers 32, and the aggregates 34 of the conductive fibers 32 can be confirmed. In fig. 4A and 4B, the fine fibrous polymer resin 33 was not observed but damaged by electron beams in SEM observation.

As described above, in order to obtain the aggregate 34 of the conductive fibers 32, the gas diffusion layer 3 contains more conductive fibers 32 than the conductive particles 31. This is because, when the amount of the conductive fibers 32 is smaller than the amount of the conductive particles 31, the conductive fibers 32 are broken by the conductive particles 31 in the mixing/dispersing step, and the aggregates 34 are hardly present.

Specific examples of the amounts of the conductive particles 31, the conductive fibers 32, and the polymer resin 33 in the gas diffusion layer 3 are as follows. That is, the gas diffusion layer 3 is preferably configured such that, for example, the conductive particles 31 are 5 wt% or more and less than 35 wt%, the conductive fibers 32 are 35 wt% or more and 80 wt% or less, and the polymer resin 33 is 10 wt% or more and 40 wt% or less.

< cumulative pore volume and pore distribution of gas diffusion layer 3 >

The pore occupying volume of the gas diffusion layer 3, that is, the cumulative pore volume of the gas diffusion layer 3 is preferably 1.0mL/g or more and 1.7mL/g or less. The reason for this is as follows. That is, when the cumulative pore volume is less than 1.0mL/g, the diffusion of gas and the discharge path of water are reduced, and the battery performance is deteriorated due to flooding. On the other hand, when the cumulative pore volume is more than 1.7mL/g, the conductivity and water retentivity are lowered, and the battery performance is lowered.

The peak of the pore size distribution of the gas diffusion layer 3 is preferably 0.05 μm or more and 0.5 μm or less. The reason for this is as follows. That is, when the peak of the pore diameter distribution is 0.05 μm or less, the pores are too small to obtain sufficient gas permeability and water drainage. On the other hand, when the peak of the pore size distribution is 0.5 μm or more, the water retentivity is lowered, the proton resistance is increased, and the battery performance is lowered particularly in low humidification.

The cumulative pore volume and pore distribution of the gas diffusion layer 3 were measured by a mercury intrusion method after drying the gas diffusion layer 3 at 120 ℃ for 4 hours as a pretreatment.

< porosity of gas diffusion layer 3 >

The porosity of the gas diffusion layer 3 is preferably 65% or more and 75% or less. The reason for this is as follows. If the porosity is less than 65%, the diffusion of gas and the discharge path of water are reduced, and the cell performance is degraded due to flooding. On the other hand, if the porosity is more than 75%, the conductivity and water retentivity decrease, and the battery performance decreases.

A method of calculating the porosity of the gas diffusion layer 3 (measurement method) will be described below. First, the apparent true density of the manufactured gas diffusion layer 3 is calculated from the true density and composition ratio of each material constituting the gas diffusion layer 3. Next, the weight, thickness, and vertical and horizontal dimensions of the manufactured gas diffusion layer 3 were measured, and the density of the manufactured gas diffusion layer 3 was calculated. Next, the calculated density and apparent true density of the gas diffusion layer 3 are substituted into an equation of porosity ═ density of the gas diffusion layer 3)/(apparent true density) × 100, and the porosity is calculated. As described above, the porosity of the manufactured gas diffusion layer 3 can be calculated.

< amount of flattening during compression of gas diffusion layer 3 >

In the manufacture of the fuel cell 100, a surface pressure of 7kgf/cm was applied²The gas diffusion layer 3 is compressed. The thickness of the gas diffusion layer 3 after compression is preferably 85% or more and 98% or less of the thickness before compression. The reason for this is as follows. That is, when the thickness of the gas diffusion layer 3 after compression is less than 85% of the thickness before compression, the rib 6 of the separator 4 applies a load during compression, and the pores in the gas diffusion layer 3 are crushed. In this case, the gas diffusion layer 3 has a portion in contact with the rib 6, which is reduced in gas diffusivity or water drainage property, and thus, the battery performance is reduced. Further, since the cross-sectional area of the fluid flow path 5 provided in the separator 4 decreases due to the gas diffusion layer 3 sagging, the pressure loss increases, or variation in pressure loss occurs depending on the degree of sagging. On the other hand, when the thickness of the gas diffusion layer 3 after compression is greater than 98% of the thickness before compression, the adhesion between the gas diffusion layer 3 and the catalyst layer 2 is reduced, a gap is formed at the interface, and the electrical conductivity is reduced, or a film of water is formed in the gap, and the gas diffusion property is reduced. Accordingly, the thickness of the gas diffusion layer 3 after compression is preferably set within the above range with respect to the thickness before compression.

< gas permeability of gas diffusion layer 3 >

The gas permeability of the gas diffusion layer 3 is preferably 5sec/100mL or more and 150sec/100mL or less. The reason for this is as follows. That is, when the gurley number is less than 5sec/100mL, the water retentivity particularly in low humidification decreases, the proton resistance increases, and the cell performance decreases. On the other hand, if the gurley number is more than 150sec/100mL, the gas permeability and the water discharge property become insufficient, and the battery performance is degraded.

The gurley number is a numerical value measured as follows. When the inner cylinder is put into the outer cylinder filled with oil, the inner cylinder gradually descends due to the weight of the inner cylinder, and the gas in the inner cylinder is compressed. At this time, a gas of a predetermined volume (100mL) is measured and passed through the outer tubeThe time required for the test piece in (2) is defined as the gurley number. The area of the air-permeable sample was 6.42cm²。

< tensile breaking Strength of gas diffusion layer 3 >

The tensile breaking strength of the gas diffusion layer 3 is preferably 0.05N/mm²the above. This is because the tensile breaking strength in the gas diffusion layer 3 is less than 0.05N/mm²In the case of (3), it is difficult to treat the gas diffusion layer 3 as a self-supporting film. The self-supporting film is a film having a self-supporting structure. Therefore, the porous member as the gas diffusion layer 3 preferably has a tensile breaking strength of 0.05N/mm²The self-supporting film described above is supported only by the conductive particles 31, the conductive fibers 32, and the polymer resin 33.

< thickness of gas diffusion layer 3 >

The thickness of the gas diffusion layer 3 is preferably 70 μm or more and 200 μm or less. The reason for this is as follows. That is, when the thickness is smaller than 70 μm, the strength of the gas diffusion layer 3 becomes weak, and it is difficult to handle the gas diffusion layer as a self-standing film. When the thickness of the gas diffusion layer 3 is larger than 200 μm, proton resistance increases, and battery performance decreases.

< method for producing gas diffusion layer 3 >

Next, a method for manufacturing the gas diffusion layer 3 according to the embodiment of the present disclosure will be described. Fig. 5 is a flowchart for explaining a method of manufacturing the gas diffusion layer 3.

In step S1, conductive particles 31, conductive fibers 32, polymer resin 33, a surfactant, and a dispersion solvent are kneaded. Specifically, a carbon material as the conductive particles 31, carbon nanotubes as the conductive fibers 32, a surfactant, and a dispersion solvent are charged, stirred, and kneaded. Then, the polymer resin 33 is put in, and stirred and kneaded again to obtain a kneaded product.

For the kneading of the material in step S1, for example, a planetary mixer, a mixing mixer, a kneader, a roll mill, or the like can be used. In step S1, which is a kneading step, the conductive particles 31 other than the polymer resin 33, the conductive fibers 32, the surfactant, and the dispersion solvent are kneaded and dispersed first, and then the polymer resin 33 is finally put into and stirred, whereby the polymer resin 33 can be uniformly dispersed in the kneaded product.

In step S2, the kneaded product is rolled and stretched into a sheet shape. In the rolling in step S2, for example, a rolling mill may be used. For example, by rolling 1 or more times under the condition of 0.001to 4ton/cm, the polymer resin 33 can be appropriately fiberized to obtain the gas diffusion layer 3 having high strength.

In step S3, the kneaded product stretched into a sheet shape is fired, and the surfactant and the dispersion solvent are removed from the kneaded product.

In the firing in step S3, for example, an IR furnace, a hot air drying furnace, or the like may be used. The firing temperature is set to a temperature higher than the temperature at which the surfactant is decomposed and lower than the temperature at which the polymer resin 33 melts. The reason for this is as follows. That is, when the firing temperature is lower than the temperature at which the surfactant is decomposed, the surfactant remains inside the gas diffusion layer 3, and the gas permeability of the gas diffusion layer 3 decreases because the inside of the gas diffusion layer 3 is hydrophilized and water remains. On the other hand, when the firing temperature is higher than the melting point of the polymer resin 33, the polymer resin 33 melts, and thus the strength of the gas diffusion layer 3 decreases. Specifically, for example, when the polymer resin 33 is PTFE, the firing temperature is preferably 280 to 340 ℃.

In step S4, the sheet-like kneaded product from which the surfactant and the dispersion solvent have been removed is re-rolled by a roll press to adjust the thickness. Thereby, the gas diffusion layer 3 according to the embodiment of the present disclosure can be manufactured.

In the re-rolling at step S4, for example, a roll press may be used. For example, the thickness and porosity of the gas diffusion layer 3 can be adjusted by performing 1 or more times of re-rolling under the condition of 0.01ton/cm to 4ton/cm as a rolling pressure.

As in the above-described examples, the present disclosure is not limited to the above-described embodiments, and can be implemented in various other embodiments.

< modification of gas diffusion layer >

As a modification of the gas diffusion layer 3 described above, the gas diffusion layer 3M will be described. Fig. 6A is a cross-sectional view showing a schematic configuration of a gas diffusion layer 3M as a modification. Fig. 6B is an enlarged cross-sectional view showing a gas diffusion layer 3M as a modification.

As shown in fig. 6A and 6B, the gas diffusion layer 3M is composed of a porous member 30 mainly composed of conductive particles 31, conductive fibers 32, and a fibrous polymer resin 33F which is a fine fibrous polymer resin. The porous member 30 has therein aggregates 34 of the conductive fibers 32 and a particulate polymer resin 33P which is a particulate polymer resin. That is, the gas diffusion layer 3M shown in fig. 6A and 6B is different from the gas diffusion layer 3M shown in fig. 3A and 3B in that the fibrous polymer resin 33F in a fibrous form and the particulate polymer resin 33P in a particulate form are polymer resins.

In this modification, PTFE (polytetrafluoroethylene) is used as the material of the fibrous polymer resin 33F and the particulate polymer resin 33P. PTFE is known to have a property of becoming fine fibers when a shearing force is applied. In the mixing, dispersing and sheeting steps in the production of the gas diffusion layer 3M, the PTFE as the material is subjected to a shearing force, whereby the PTFE is formed into a fine fibrous form. Thereby, the fibrous polymer resin 33F is formed inside the gas diffusion layer 3M (porous member 30).

the fibrous polymer resin 33F functions as a binder for binding conductive particles and conductive fibers, and thus the gas diffusion layer 3M can have a self-supporting structure.

On the other hand, the particulate polymer resin 33P is present as particles without being fibrillated inside the gas diffusion layer 3M. The PTFE in the form of particles has higher hydrophobicity in the vicinity of the particles than the PTFE in the form of fibers. Therefore, the water accumulated in the pores present in the gas diffusion layer 3M can be appropriately prevented from inhibiting the gas permeation by the particulate polymer resin 33P.

The particle-like polymer resin 33P has a short diameter and a long diameter both in the range of 0.1 to 10 μm. This is because particles smaller than 0.1 μm are not present at the stage of the raw material, and when the particle diameter is larger than 10 μm, the conductivity is lowered.

In any cross section of the gas diffusion layer 3M (porous member 30), the area ratio of the particulate polymer resin 33P is 0.1% or more and 1% or less. The reason for this is as follows. That is, when the area ratio of the particulate polymer resin 33P is less than 0.1%, the water-repellent effect in the interior of the gas diffusion layer 3M is small, and water is likely to accumulate in the pores in the interior. On the other hand, if the area ratio is more than 1%, the conductivity decreases, and the battery performance decreases.

The amount of the polymer resin in the gas diffusion layer 3M, which is the combination of the conductive particles 31, the conductive fibers 32, the fibrous polymer resin 33F, and the particulate polymer resin 33P, is the same as that of the gas diffusion layer 3M described above. Specifically, the conductive particles 31 may be 5 wt% or more and less than 35 wt%, the conductive fibers 32 may be 35 wt% or more and 80 wt% or less, and the polymer resin obtained by combining the fibrous polymer resin 33F and the particulate polymer resin 33P may be 10 wt% or more and 40 wt% or less. The reason for this is as follows. That is, in the case where the amount of the polymer resin is less than 10 wt%, a shearing force is applied to the entire polymer resin at the time of manufacturing the gas diffusion layer 3. In this case, almost all of the polymer resin becomes fibrous polymer resin 33F, and almost no particulate polymer resin 33P exists. On the other hand, when the amount of the polymer resin is more than 40 wt%, the proportion of the particulate polymer resin 33P increases, and the electrical conductivity of the gas diffusion layer 3 decreases.

< method for calculating area ratio of particulate Polymer resin >

The area ratio of the particulate polymer resin 33P in an arbitrary cross section of the gas diffusion layer 3M (porous member 30) is calculated as follows. First, the gas diffusion layer 3M is cut, and after polishing the cross section, a photograph of the cross section is taken by SEM or optical microscope. Then, the particle-like polymer resin 33P of 0.1 to 10 μm was selected from the cross-sectional photograph, and the area ratio was calculated from the area ratio of the particle-like polymer resin to the total cross-sectional area.

Fig. 7 is a cross-sectional SEM photograph of the gas diffusion layer 3M. The conductive particles 31, the conductive fibers 32, and the particulate polymer resin 33P were confirmed. In addition, the fine fibrous polymer resin 33F was not observed but damaged by electron beams in SEM observation.