JP6973202B2 - Manufacturing method of gas diffusion layer for fuel cell - Google Patents

Description

The present invention relates to a method for manufacturing a gas diffusion layer for a fuel cell.

A fuel cell supplies fuel gas (hydrogen gas) and oxidant gas (oxygen gas) to two electrically connected electrodes, and electrochemically causes the fuel to oxidize, thereby directly supplying chemical energy. Convert to energy. This fuel cell is usually configured by stacking a plurality of single cells having a membrane electrode assembly in which an electrolyte membrane is sandwiched between a pair of electrodes. Among them, the polymer electrolyte fuel cell using the polymer electrolyte membrane as the electrolyte membrane has advantages such as easy miniaturization and operation at a low temperature, so that it is particularly portable and mobile. It is attracting attention as a body power source.

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

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

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

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

Each electrode of the anode and the cathode generally has a structure in which a catalyst layer and a gas diffusion layer are laminated in this order from the electrolyte membrane side. The catalyst layer usually contains an electrode catalyst such as platinum or a platinum alloy for promoting the electrode reaction, a polyelectrolyte for ensuring proton conductivity, and a conductive material for ensuring electron conductivity. ing. Further, the gas diffusion layer is usually formed by using a conductive porous body capable of supplying the reaction gas to the catalyst layer, discharging 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 a wet state of the electrolyte membrane and the catalyst layer in order to ensure ionic conductivity. Therefore, in general, the reaction gas is supplied to the electrode in a pre-humidified state.

On the other hand, in a fuel cell, water is generated with power generation as described above. Most of the generated water is discharged to the outside of the cell together with the unreacted gas (reaction gas that did not contribute to the electrode reaction) discharged from the electrode and the humidified water supplied as steam. However, a part of the generated water and the humidified water supplied as steam to the electrode together with the reaction gas is condensed depending on the environment inside the fuel cell and exists in the electrode in a liquid state. At this time, if the condensed water (liquid water) stays in the electrode, the liquid water is clogged, that is, so-called flooding occurs, the supply of the reaction gas is hindered, and the power generation performance is deteriorated. In particular, when the fuel cell is operated under high humidification conditions, a large amount of liquid water is generated, so it is necessary to ensure the dischargeability of the liquid water of the electrode.

Therefore, it is desired that the liquid water in the electrode is quickly discharged to the outside of the cell to prevent the liquid water from staying in the electrode.

As a gas diffusion layer for suppressing this flooding, a porous base material layer having a porous structure and a fine porous layer having a pore diameter smaller than that of the porous base material layer (microporous layer, hereinafter also referred to as MPL). Is known to have a laminated structure. By providing this fine porous layer, it is said that the ultrafine porous structure of the fine porous layer inhibits the formation of a liquid water film and improves the drainage property of the gas diffusion layer.

One of the problems of the gas diffusion layer having such a fine porous layer is that the surface roughness of the fine porous layer is large, so that when it is incorporated into a fuel cell, the electrolyte membrane is damaged and the durability of the fuel cell is durable. The problem of reduced sex is known.

Many efforts have been made to solve the problem of surface roughness of such a fine porous layer. For example, in Patent Document 1, a coating liquid for forming a fine porous layer (hereinafter, also referred to as MPL coating liquid) containing carbon particles having a specific range of dibutylphthalate (DBP) oil absorption is applied onto a carbon sheet. A method for producing a gas diffusion layer including the above has been proposed.

International Publication No. 2016/076132

According to the method for producing a gas diffusion layer described in Patent Document 1, by increasing the amount of DBP oil absorption of carbon for forming MPL, the permeation of the MPL coating liquid into the carbon sheet is suppressed, and this is suppressed. It is said that the surface roughness of MPL is reduced by suppressing the penetration. However, in the method for producing a gas diffusion layer described in Patent Document 1, since the secondary particle size of the carbon particles in the MPL coating liquid becomes large, the infiltration of the MPL coating liquid into the carbon sheet is suppressed. However, there is a problem that it is difficult to sufficiently reduce the surface roughness of the MPL.

The present invention has been made in view of the above circumstances, and is a gas diffusion layer that suppresses the infiltration of the MPL coating liquid into the inside of the porous base material layer and sufficiently reduces the surface roughness of the MPL. It is an object of the present invention to provide the manufacturing method of.

The present invention achieves the above object by the following means.

A method for manufacturing a gas diffusion layer for a fuel cell, which comprises a porous base material layer and a fine porous layer.
By crimping a polymer sheet to one surface of the porous substrate layer,
Applying the coating liquid for forming the fine porous layer onto the polymer sheet,
The porous base material layer coated with the coating liquid for forming the fine porous layer is heated and fired.
A method for producing a gas diffusion layer for a fuel cell, wherein the polymer sheet is a sheet made of a polymer that volatilizes by firing.

According to the method for producing a gas diffusion layer for a fuel cell of the present invention, the MPL coating liquid is made porous by applying the MPL coating liquid after crimping a polymer sheet to one surface of the porous base material layer. It is possible to suppress the penetration into the base material layer and sufficiently reduce the surface roughness of the MPL.

It is explanatory drawing which shows the manufacturing method of the gas diffusion layer of this invention. It is an SEM photograph of the cross section of the gas diffusion layer obtained by the method of this invention. It is an SEM photograph of the cross section of the gas diffusion layer obtained in the comparative example.

The method for producing a gas diffusion layer of the present invention is
To crimp a polymer sheet to one surface of the porous substrate layer,
Applying a coating liquid for forming a fine porous layer on the polymer sheet,
The porous base material layer coated with the coating liquid for forming the fine porous layer is heated and fired.
The polymer sheet is a polymer sheet that volatilizes by firing.

The carbon fiber generally used as the porous base material of the gas diffusion layer has a pore diameter of 1 to 50 μm, whereas the primary particle size of the conductive fine particles constituting the MPL, for example, the carbon particles is usually 0. .1 μm or less. When the secondary particle size of the conductive fine particles is, for example, 50 μm or more, it is possible to suppress the infiltration of the conductive fine particles into the porous substrate layer in the MPL coating liquid at the time of forming the MPL. can. However, in this case, the surface roughness of the MPL formed deteriorates due to the increase in the secondary particle size of the conductive fine particles. On the contrary, when the secondary particle size of the conductive fine particles is reduced to 1 μm or less, the conductive fine particles in the MPL coating liquid permeate into the porous base material layer at the time of forming the MPL.

Further, when controlling the secondary particle size of the conductive fine particles to a predetermined size, it is necessary to control the secondary particle size of the conductive fine particles in the solvent when applying the MPL coating liquid. However, in the preparation of the MPL coating liquid, it is difficult to make the secondary particle size of the conductive fine particles uniform because the conductive fine particles are mechanically dispersed using a mixer or a stirrer.

Further, in the MPL coating liquid, conductive fine particles and a fluorine-based resin for imparting a water-repellent function may be mixed. In this case, the conductive fine particles and the fluorine-based resin are uniformly dispersed. Producing secondary particles is even more difficult.

Further, when the MPL coating liquid is applied to the porous base material layer, a method such as a die coater or a gravure coating is generally used, and in this case, it is necessary to control the viscosity of the MPL coating liquid. However, by controlling the secondary particle size of the conductive fine particles in the MPL coating liquid, the viscosity of the MPL coating liquid may change, and the coatability of the MPL coating liquid may deteriorate.

On the other hand, according to the method of the present invention, a polymer sheet that volatilizes by firing is pressure-bonded onto one surface of the porous substrate layer, and then the MPL coating liquid is applied onto the polymer sheet. ing. As a result, the infiltration of the MPL coating liquid into the porous base material layer is suppressed, the secondary particle size of the conductive fine particles in the MPL coating liquid can be reduced, and the surface roughness of the MPL can be reduced. Can be made small enough.

Hereinafter, embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments, and can be variously modified and implemented within the scope of the gist of the present invention.

The method for producing the gas diffusion layer for a fuel cell of the present invention is, for example, as shown in FIG. 1, a polymer sheet crimping step (step 1), a coating liquid coating step for forming a fine porous layer (step 2), and a firing step. (Step 3) is executed in order. Hereinafter, each step of the method for manufacturing the gas diffusion layer for a fuel cell of the present invention will be described.

<Polymer sheet crimping process (process 1)>
In the polymer sheet crimping step, the polymer sheet 12 is crimped to one surface of the porous base material layer 11.

Here, the porous base material layer 11 is a sheet-like material having conductivity and porosity generally used as a base material of a gas diffusion layer of a fuel cell, for example, carbon paper, carbon cloth, or carbon. A porous sheet material made of carbon fiber such as a non-woven fabric can be used.

The porous base material layer 11 may contain a water repellent. This water-repellent agent is contained in the porous base material layer 11 by, for example, applying a water-repellent treatment coating liquid containing a water-repellent agent to one surface of the porous base material layer 11 and then drying the mixture. Can be made. This water-repellent coating coating liquid is a dispersion liquid of a water-repellent agent. As the water repellent, fluorine-based polymer materials such as polytetrafluoroethylene, polyvinylidene fluoride, polyhexafluoropropylene, and tetrafluoroethylene-hexafluoropropylene copolymer, polypropylene, polyethylene, and the like can be used. Among these materials, a fluorine-based polymer material, particularly polytetrafluoroethylene, is preferably used.

The polymer sheet 12 is a polymer sheet that volatilizes in the subsequent firing step. As this polymer, polyesters such as polyethylene terephthalate, polyethylene naphthalate, and polytributylene terephthalate, and fluororesins such as polytetrafluoroethylene and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers can be used. .. Further, the polymer sheet 12 is porous, and in particular, its pore diameter is 0.05 to 1 μm, its thickness is 1 to 100 μm, its melting point is 40 to 400 ° C., and its surface is hydrophilic. It is preferable that the polymer is used.

<Coating liquid coating process for forming a fine porous layer (process 2)>
In the coating liquid for forming a fine porous layer (MPL coating liquid), the MPL coating liquid 13 is applied onto the polymer sheet 12 pressure-bonded onto the porous base material layer 11.

This MPL coating liquid coating step can be performed using various coating devices, but it is preferable to use a die-type coater. This die-type coater may include a die head, and the die head may be filled with an MPL coating liquid. The basis weight of the MPL coating liquid is, for example, ² to 6 mg / cm 2.

In the conventional method for producing a gas diffusion layer, the MPL coating liquid is directly applied to one surface of the porous base material layer to form MPL. In this case, since the porous base material layer is porous, it was not possible to avoid the infiltration of the MPL coating liquid into the porous base material layer. In the present invention, the polymer sheet is pressure-bonded onto the porous base material layer, and the MPL coating liquid is applied onto the polymer sheet. Therefore, the MPL coating liquid is impregnated into the porous base material layer. Can be avoided.

Here, the MPL coating liquid 13 may be in the form of a paste or a slurry in which conductive particles, a binder and a solvent are mainly mixed and dispersed. Additives such as a dispersant can be added to the MPL coating liquid 13 as needed, but it is preferable that the MPL coating liquid 13 does not contain a metal in order to avoid contamination.

As the conductive particles, carbon particles, particularly carbon particles having an average particle size of 20 to 150 nm, for example, carbon black having excellent conductivity and a large specific surface area can be used, and acetylene black having high conductivity is particularly preferable. .. As the binder, a fluorine-based polymer material such as polytetrafluoroethylene, polyvinylidene fluoride, polyhexafluoropropylene, or tetrafluoroethylene-hexafluoropropylene copolymer, polypropylene, polyethylene, or the like can be used. Among these materials, a fluorine-based polymer material, particularly polytetrafluoroethylene, is preferably used. The solvent is not particularly limited, and various liquids such as water, methanol, and ethanol can be used. The surfactant as a dispersant is also not particularly limited, and various surfactants such as various nonionic surfactants such as an ester type, an ether type, and an ester ether type can be used.

The composition of the MPL coating liquid 13 is, for example, 70 to 90% by mass of the conductive particles, 15 to 25% by mass of the binder, and 15 to 25% by mass of the dispersant, with the total solid content of the conductive particles, the binder and the dispersant being 100% by mass. It may be adjusted to be 5 to 15% by mass. The physical properties of the MPL coating liquid 13 are as follows: a solid content ratio of 15 to 25% by mass, a viscosity of 500 to 2500 mPa · s (50 / s) at a ^{shear rate of 50 s -1, and a storage elastic modulus of 500 to 5500 Pa.} It may be set to be.

<Baking step (step 3)>
In the firing step, the porous base material layer 11 coated with the MPL coating liquid 13 on the polymer sheet 12 is heated and fired.

This firing step may be performed in a heating furnace generally used in the firing step. In the firing step, the porous base material layer 11 on which the MPL coating liquid 13 is applied on the polymer sheet 12 is heated by a heater, so that the coating film is fired by the MPL coating liquid 13. At this time, since the polymer sheet 12 is composed of the polymer volatilized by this firing, the polymer sheet 12 is volatilized by firing, and the coating film of the MPL coating liquid 13 is fixed on the porous base material layer 11 as MPL 14. A gas diffusion layer in which the porous base material layer 11 and the MPL 14 are laminated is formed.

The firing temperature is a temperature for heat-sealing the conductive particles and the binder in the MPL coating liquid 12, and is preferably set to a temperature of 250 ° C. or higher, more preferably 400 ° C. or higher. The upper limit of the heating temperature is not particularly limited. Further, the firing time may be set to an appropriate time, for example, about 1 minute to 120 minutes, depending on the coating amount of the MPL coating liquid, the firing temperature, and the like. The upper limit of the heating time is also not particularly limited.

<Example 1>
A 2 cm x 2 cm carbon fiber sheet (Toray, TGP-H060) and a 2 cm x 2 cm polymer sheet (Merck, Omnipore Membrane Filter JGWP14225) are stacked, sandwiched between SUS flat plates, and at 3 MPa for 10 minutes. Pressed. The physical characteristics of this polymer sheet were pore diameter: 0.2 μm, thickness: 80 μm, material: polytetrafluoroethylene, and porosity: 80%.

The MPL slurry containing 7.2 wt% of acetylene black, 1.8 wt% of polytetrafluoroethylene, 90 wt% of water and 1 wt% of the dispersant was sucked up with a dropper, and a few drops of carbon fiber crimped with the polymer sheet were dropped onto the polymer sheet. ..

The carbon fiber coated with the MPL slurry was set in a tubular furnace, and the temperature was raised from room temperature to 360 ° C. over 30 minutes while flowing argon gas in the furnace at 0.2 L / min, and the temperature was maintained at 360 ° C. for 15 minutes. After that, the heat history of furnace cooling was added. After the temperature in the furnace reached room temperature, the carbon fiber-MPL laminate was taken out from the furnace.

<Comparative Example 1>
A carbon fiber-MPL laminate was obtained in the same manner as in Example 1 except that a polymer sheet was not used.

SEM images of cross sections were taken for each of the laminates obtained in Example 1 and Comparative Example 1. The results are shown in FIGS. 2 and 3. It was shown that the infiltration of MPL into the carbon fibers was suppressed in the laminate of Example 1 as compared with the laminate of Comparative Example 1.

11 Porous base material layer 12 Polymer sheet 13 Coating liquid for forming a fine porous layer 14 Fine porous layer

Claims (1)

A method for manufacturing a gas diffusion layer for a fuel cell, which comprises a porous base material layer and a fine porous layer.
By crimping a polymer sheet to one surface of the porous substrate layer,
Applying fine fine porous layer forming coating solution onto the polymer sheet,
The porous base material layer coated with the coating liquid for forming the fine porous layer is heated and fired.
A method for producing a gas diffusion layer for a fuel cell, wherein the polymer sheet is a sheet made of a polymer that volatilizes by firing.

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