JP5079195B2 - Gas diffusion electrode for fuel cell and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a gas diffusion electrode of a polymer electrolyte fuel cell using hydrogen, methanol, ethanol, dimethyl ether or the like as a fuel and air or oxygen as an oxidant, and a method for producing the same.
[0002]
[Prior art]
A gas diffusion electrode of a polymer electrolyte fuel cell is generally composed of a catalyst layer sandwiching a polymer electrolyte membrane and a porous substrate in contact with the catalyst layer. With one function.
The first is a function of diffusing these gases in order to uniformly supply fuel gas or oxidant gas from the gas flow channel located on the outer surface of the gas diffusion electrode to the catalyst in the catalyst layer. The second is a function of quickly discharging water generated by the electrode reaction in the catalyst layer to the gas flow path. The third function is to conduct electrons exchanged with the electrode reaction. Therefore, the porous substrate needs to have high gas permeability, water vapor permeability, and electronic conductivity.
[0003]
From the viewpoint of improving water vapor permeability, studies have been made to suppress water retention (flooding) by dispersing a water-repellent polymer typified by a fluororesin in a porous substrate.
For example, JP-A-6-203851, JP-A-7-130373, JP-A-8-106915, or JP-A-9-259893 discloses polytetrafluoroethylene (hereinafter abbreviated as PTFE) or tetrafluoro. A method of impregnating and drying carbon paper or carbon cloth in a dispersion of a copolymer of ethylene and hexafluoropropylene (hereinafter abbreviated as FEP) is disclosed.
JP-A-7-220734, JP-A-4-67571, JP-A-3-208260, JP-A-3-208261, JP-A-3-208262, or JP-A-6-44984 Discloses a porous substrate on which a layer made of fine carbon powder to which PTFE is added is formed.
[0004]
However, the method of randomly impregnating and drying carbon paper or carbon cloth in a water-repellent polymer dispersion causes the water-repellent polymer to be distributed according to the fiber arrangement of the porous substrate having a three-dimensional structure. Therefore, it is difficult to control the distribution of the water repellent material. Further, in inverse proportion to the distribution of the voids in the porous substrate, the water repellent material does not collect at the large void portions, and the water repellent material tends to collect at the small void portions. Further, in the above impregnation method, too much water repellent material is attached to the surface of the porous base material, water is trapped inside the base material, and flooding is caused. As a result, the discharge characteristics and reliability of the fuel cell also deteriorate.
[0005]
In addition, when water-repellent polymer is added to the porous substrate to increase water vapor permeability and the distribution of the water-repellent polymer in the substrate is not controlled, there is a problem that gas permeability and electronic conductivity are lowered. is there. Therefore, instead of configuring the porous base material from a single base material, there is an effort to combine carbon fiber layers with carbon powder and water-repellent polymer layers to achieve conflicting functions. Although it has been done, sufficient results have not been obtained.
[0006]
[Problems to be solved by the invention]
In order to improve the water vapor permeability of the gas diffusion electrode and ensure the gas permeability, it is considered essential to appropriately control the distribution of the water repellent material in the gas diffusion electrode. The present invention solves this problem, suppresses flooding, ensures water vapor permeability and gas permeability, and provides a fuel cell with high discharge performance and reliability.
[0007]
[Means for Solving the Problems]
The present invention comprises a catalyst layer comprising a carbon powder carrying a catalyst and a polymer electrolyte, a porous substrate made of a carbon material and in contact with the catalyst layer, and a water repellent material applied to the porous substrate. A gas diffusion electrode, wherein the amount of the water repellent material in the porous substrate is continuously reduced from the side in contact with the catalyst layer toward the other side. The present invention relates to a gas diffusion electrode.
[0008]
Ie, when the assembled fuel cell, you decrease the water repellency of the porous substrate from the polymer electrolyte membrane side to the separator side.
The catalyst layer that contains a water repellent. When the catalyst layer has a thickness of Wmm and contains Xg of water repellent material per cm ² , and the porous substrate contains a thickness of Zmm and Yg of water repellent material per cm ² (X / W)> to meet the (Y / Z).
[0009]
The present invention also provides (1) a dispersion of a water-repellent polymer or a solution of a water-repellent polymer on one surface of a Zmm-thick porous substrate made of a carbon material, with Yg of the above-mentioned repellent per 1 cm ^2. Step 1 for applying the aqueous polymer so that the porous substrate contains it, and (2) On the surface thereof, a carbon repellent carrying a catalyst and a polymer electrolyte, and Xg of water repellent material per 1 cm ^2. The present invention relates to a method for producing a gas diffusion electrode for a fuel cell, comprising a step 2 of forming a catalyst layer having a thickness of Wmm so as to satisfy (X / W)> (Y / Z) .
Prior to step 1, it is preferable to perform a step of heating the porous substrate to 40 to 180 ° C.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The gas diffusion electrode for a fuel cell of the present invention comprises a catalyst layer comprising a carbon powder carrying a catalyst and a polymer electrolyte, a porous substrate made of a carbon material and in contact with the catalyst layer, and a repellent applied to the porous substrate. Consists of water material. In the fuel cell, the catalyst layer side of the gas diffusion electrode is in contact with the polymer electrolyte membrane, and the porous substrate side is in contact with the separator. The porous substrate constitutes a gas diffusion electrode substrate having a role of diffusing fuel gas and oxidant gas.
[0011]
FIG. 1 shows a basic structure of a fuel cell including the gas diffusion electrode. The polymer electrolyte membrane 11 is sandwiched between gas diffusion electrodes 14 including a catalyst layer 12 and a gas diffusion electrode base material 13. A joined body of the polymer electrolyte membrane 11 and the gas diffusion electrode 14 is called an MEA (membrane-electrode assembly) 15. The MEA 15 is sandwiched by a separator 17 having a fuel gas or oxidant gas flow path 16.
[0012]
In the above basic structure, the fuel gas is supplied to the gas diffusion electrode base material from the fuel gas flow path of the anode separator, passes through the electrode base material while diffusing, and reaches the catalyst layer. The oxidant gas is supplied to the electrode base material from the oxidant gas flow path of the cathode-side separator, passes through the electrode base material while diffusing, and reaches the catalyst layer.
[0013]
The electrode reaction occurs on the surface of the catalyst contained in the catalyst layer 12. In the catalyst layer on the anode side, a reaction of H ₂ → 2H ⁺ + 2e ⁻ occurs. In the catalyst layer on the cathode side, a reaction of 1 / 2O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O occurs. The overall reaction is H ₂ + 1 / 2O ₂ → H ₂ O + Q. An electromotive force is obtained by this reaction, and power generation is possible. At the same time, water is generated in the catalyst layer on the cathode side. Further, H ⁺ generated in the catalyst layer on the anode side during the reaction moves in the polymer electrolyte membrane and reaches the catalyst layer on the cathode side. At this time, one H ⁺ ion moves along with 5 to 20 H ₂ O molecules.
[0014]
The polymer electrolyte membrane exhibits high hydrogen ion conductivity for the first time in a state swollen with a sufficient amount of water. However, since a large amount of water moves with the H ⁺ ions moving in the polymer electrolyte membrane to the cathode, it is necessary to always supply water to the polymer electrolyte membrane. This water is supplied as water vapor from the gas flow path to the gas diffusion electrode substrate, and then supplied to the polymer electrolyte membrane through the cathode and the anode. In addition, of the water generated in the catalyst layer on the cathode side, surplus moisture that is not required by the polymer electrolyte membrane is discharged to the outside through the gas diffusion electrode substrate.
[0015]
As described above, since the gas diffusion electrode has a large amount of water in and out, it is important to control the water repellency in the gas diffusion electrode. In particular, from the viewpoint of ensuring reliability, it is necessary to design water repellency so that excess water can be quickly discharged to the outside.
[0016]
The amount of the water repellent material in the porous substrate continuously decreases from the side in contact with the catalyst layer toward the other side. By providing the water repellent material with an inclination in this way, a gas diffusion path and a water movement path from the polymer electrolyte membrane side to the separator side are formed. As a result, it is possible to provide a highly reliable fuel cell that is free from water clogging and has excellent discharge performance.
[0017]
The amount of the water repellent materials in the porous base in the material, to reduce the side in contact with the catalyst layer toward the other side, because the water repellency will be reduced along the discharge direction of the water, the movement direction of the water Is more reliably controlled, and a great effect of suppressing water clogging in the porous substrate can be obtained.
[0018]
The catalyst layer in contact with the porous substrate, at a higher rate than the addition ratio of the water repellent material to the porous substrate, to grant water repellent. That is, when the catalyst layer has a thickness of Wmm and contains X mg of a water repellent material per 1 cm ² , and the porous substrate has a thickness of Z mm and contains 1 mg of water repellent material per 1 cm ² ( X / W)> to meet the (Y / Z). Further, 0.008 ≦ X (mg) ≦ 1.3, and 0.24 ≦ Y (mg) ≦ 10 is preferable.
[0019]
By imparting high water repellency to the catalyst layer that generates water in this way, excess water quickly moves to a gas diffusion electrode substrate or polymer electrolyte membrane having lower water repellency. In the electrode base material, the water repellent material is distributed so that the water repellency decreases toward the gas flow path, so that the water quickly moves to the gas flow path. Thus, excess water is quickly discharged to the outside by controlling water repellency, and flooding due to water clogging in the gas diffusion electrode can be suppressed. Further, since the gas permeability is not lowered, a fuel cell having high discharge characteristics and high reliability can be obtained.
[0020]
In the gas diffusion electrode of the present invention, (1) a water-repellent polymer dispersion or a water-repellent polymer solution is applied to one surface of a Zmm-thick porous substrate made of a carbon material at a rate of Yg / cm ² . Step 1 for coating the water-repellent polymer so that the porous substrate contains it, and (2) water-repellent Xg / cm ² consisting of carbon powder supporting the catalyst and polymer electrolyte on its surface. It can manufacture by performing the process 2 which forms the catalyst layer of thickness Wmm containing a material so that (X / W)> (Y / Z) may be satisfy | filled.
By applying a dispersion or solution (hereinafter referred to as a water repellent liquid) only to one surface of a porous substrate arranged almost horizontally, it is easily repelled in a direction perpendicular to the surface of the porous substrate. The distribution of the water material can be continuously changed.
[0021]
As the water repellent polymer, it is preferable to use a fluorine resin, a silicone resin, or the like. As the fluororesin, PTFE, FEP, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-ethylene copolymer, polychlorotrifluoroethylene, polyvinylidene fluoride, or the like can be used. As the silicone resin, polydimethylsiloxane, polymethylhydrosiloxane, polyphenylhydrosiloxane, or the like can be used.
Carbon paper, carbon cloth, carbon nonwoven fabric, etc. can be used for the porous substrate.
[0022]
Prior to step 1, by performing the step of heating the porous substrate, it is possible to control the state of penetration of the water repellent material into the porous substrate. The higher the temperature of the porous substrate, the faster the drying / evaporation rate of the solvent or dispersion medium when the water-repellent liquid comes into contact with the porous substrate. Can be reduced. In addition, as the polymer concentration of the water repellent liquid increases, the amount of the solvent or dispersion medium decreases, so the drying / evaporation time of the solvent or dispersion medium when the water repellent liquid contacts the porous substrate is shortened. The amount of water-repellent polymer that permeates into the porous substrate can be reduced. In order to control the penetration state of the water repellent material into the porous base material to the most suitable state, it is effective to heat the porous base material to 40 to 180 ° C, preferably 60 to 80 ° C.
[0023]
For application of the water repellent liquid, for example, a spray method is preferably employed. At that time, by controlling the permeation state of the water repellent material into the porous substrate by the water repellent polymer concentration of the water repellent liquid used for coating, the discharge amount of the liquid from the spray nozzle, the atomization pressure, etc. The distribution of the water repellent material in the porous substrate can be controlled. Further, the permeation state of the water repellent material into the porous substrate also depends on the distance between the porous substrate and the spray nozzle, the atmospheric temperature and humidity during coating, the temperature of the water repellent liquid, and the like.
[0024]
When the discharge amount of the water repellent liquid sprayed from the spray nozzle is reduced and the atomization pressure is increased, the water repellent polymer comes into contact with the porous substrate with the dispersion medium and solvent almost evaporated. The amount of water repellent polymer that penetrates into the interior of the material is reduced. Conversely, when the amount of water-repellent liquid sprayed from the spray nozzle is increased and the atomization pressure is reduced, the wet water-repellent polymer comes into contact with the porous substrate. The amount of the water repellent polymer that permeates increases.
[0025]
For the application of water repellent liquid, it is easy to control the concentration of the water repellent liquid and the porous substrate, and the spray method with simple operation seems to be effective, but the same control is possible with any other method. Is possible and can be applied to the present invention. For example, in the doctor blade method, the screen printing method, and the coater coating method, the penetration state of the water repellent material into the porous substrate is most suitable by controlling the concentration of the water repellent liquid to 60 to 80% by weight. The state can be controlled.
Note that it is not necessary to add a desired amount of the water-repellent material to the porous substrate in a single coating, and a plurality of coatings may be performed.
[0026]
【Example】
<< Reference Example 1 >>
A Daikin Industries FEP dispersion (trade name: ND-1) was diluted with water so that the weight ratio of ND-1 to water was 1:10. As the porous substrate, carbon paper (trade name: TGP-H-120, thickness 0.36 mm) manufactured by Toray Industries, Inc. was used.
This carbon paper was placed on a hot plate and heated to 60 ° C. from one side. Then, the FEP dispersion diluted from the spray nozzle was applied to the other surface of the carbon paper. At this time, the discharge rate of the dispersion liquid from the spray nozzle was 30 cc / min, and the atomization pressure was 1.5 kg / cm ² . The distance from the spray nozzle to the carbon paper was 200 mm, the ambient temperature was adjusted to 20 ° C., and the humidity was adjusted to 30%.
[0027]
Immediately after the diluted dispersion of FEP was applied, the carbon paper was turned over so that the FEP coated surface faced down and dried in an atmosphere of 60 ° C. for 2 hours. Thereafter, the carbon paper was baked at about 380 ° C. for 15 minutes. The amount of FEP in the carbon paper after firing was the largest in the vicinity of the FEP coated surface, and gradually decreased toward the opposite surface. Here, the amount of FEP imparted to the carbon paper was 1.3 mg per 1 cm ^{2 of} the carbon paper.
[0028]
Next, catalyst layers were formed on both sides of the polymer electrolyte membrane. A Nafion 112 membrane manufactured by DuPont USA was used as the polymer electrolyte membrane. The catalyst was prepared by supporting platinum on Ketjen Black EC, a carbon fine powder manufactured by Lion Corporation. Here, 50 parts by weight of platinum was supported on 50 parts by weight of ketjen black EC. A catalyst composition was prepared by mixing 500 parts by weight of a polymer electrolyte dispersion (Nafion solution) manufactured by DuPont, USA with 100 parts by weight of the obtained catalyst. Using this catalyst composition, a catalyst layer having a thickness of 0.02 mm was formed on both sides of the Nafion 112 membrane. And it heat-welded at 160 degreeC, the catalyst layer and the Nafion112 film | membrane were joined, and MEA was produced. Nafion is a trade name of perfluorocarbon sulfonic acid.
[0029]
Carbon paper to which FEP was imparted as described above was joined to this MEA so that the FEP coated surface was in contact with the catalyst layer, thereby producing a unit cell A of a hydrogen-air type fuel cell.
[0030]
Example 1
A unit cell B of a hydrogen-air type fuel cell was produced in the same manner as in Reference Example 1 except that 500 parts by weight of the Nafion solution was added to 100 parts by weight of the catalyst and 20 parts by weight of PTFE was further mixed. Here, the amount of PTFE applied to the catalyst layer was 0.1 mg per 1 cm ^{2 of} the catalyst layer.
[0031]
<< Comparative Example 1 >>
An MEA similar to that of Example 1 was produced. Carbon paper to which FEP was imparted in the same manner as in Reference Example 1 was joined to this MEA so that the surface opposite to the FEP-coated surface was in contact with the catalyst layer, thereby producing a unit cell C of a hydrogen-air type fuel cell. .
[0032]
<< Comparative Example 2 >>
A Daikin Industries FEP dispersion (trade name: ND-1) was diluted with water so that the weight ratio of ND-1 to water was 1:10. A carbon paper (trade name: TGP-H-120, product thickness: 0.36 mm) manufactured by Toray Industries, Inc. was immersed in this diluted FEP dispersion for 1 minute and then pulled up. After drying for a period of time, the carbon paper was fired at about 380 ° C. for 15 minutes. The water repellent material in the carbon paper after firing was uniformly distributed in the carbon paper. Moreover, the amount of FEP imparted to the carbon paper here was 1.3 mg per 1 cm ^{2 of} the carbon paper.
[0033]
On the other hand, the same MEA as in Reference Example 1 was produced. Carbon paper to which FEP was applied by dipping was joined to this MEA to produce a unit cell D of a hydrogen-air type fuel cell.
[0034]
Reference Example 1 prepared as described above, the unit cells A of Comparative Examples 1 and 2 and Contact Example 1, B and C, the pure hydrogen gas to the fuel electrode and D, by supplying air respectively to the air electrode, the unit cell The discharge test was conducted. At that time, the battery temperature was 75 ° C., the fuel gas utilization rate was 70%, and the air utilization rate Uo was 40%. Gas humidification was performed by passing fuel gas and air through a bubbler at 70 ° C., respectively.
[0035]
FIG. 2 shows the relationship between the battery voltage and the current density of the unit cell. The cell voltages of the cells A, B, C, and D at a current density of 800 mA / cm ² were 635 mV, 645 mV, 480 mV, and 530 mV, respectively. As can be seen from FIG. 2, the higher the current density, the greater the difference in discharge characteristics. This result was thought to be due to the fact that as the current density increased, the amount of water produced from the battery increased in proportion to this, and the influence of the distribution of the water repellent material in the electrode increased. That is, in the unit cell B in which the amount of the water repellent material in the vicinity of the polymer electrolyte membrane is the largest and the amount of the water repellent material gradually decreases along the excess water discharge path, the water does not cause partial stagnation or flooding. It is considered that good discharge performance was exhibited without reducing gas diffusivity.
[0036]
On the contrary, in the single cells C and D in which the water repellency in the gas diffusion electrode does not decrease along the water discharge direction, flooding occurred in the gas diffusion electrode, and the gas permeability was hindered. It was considered inferior. In the cell C, flooding occurred between the catalyst layer and the gas diffusion electrode substrate and on the catalyst layer side in the electrode substrate, and in the cell D, it was considered that flooding occurred in the entire electrode substrate. .
[0037]
Next, pure hydrogen gas was supplied to the fuel electrode of each of the single cells A and B of Reference Example 1 and Example 1, and the single cells C and D of Comparative Examples 1 and 2, and air was supplied to the air electrode. A durability test was conducted. At that time, the battery temperature was 75 ° C., the fuel gas utilization rate was 70%, and the air utilization rate Uo was 40%. The current density was 0.3 A / cm ² and gas humidification was performed by passing fuel gas and air through a 70 ° C. bubbler.
[0038]
FIG. 3 shows the relationship between the battery voltage of the single cell and the operation time. As can be seen from the results of FIG. 3, in the cells A and B in which the amount of water repellent material in the vicinity of the polymer electrolyte membrane is large and the amount of water repellent material gradually decreases along the excess water discharge path, Since it does not cause flooding, high durability was obtained.
[0039]
On the contrary, in the unit cells C and D in which the water repellency inside the gas diffusion electrode does not decrease along the direction of moisture discharge, flooding occurred in the gas diffusion electrode and gas permeability was hindered. It became low.
[0040]
A fuel cell is usually used by connecting a plurality of single cells in series or in parallel. Therefore, the flooding of the single cell greatly affects the performance of the fuel cell stack. In particular, when single cells are connected in series, the limit current value of the unit cell with the lowest characteristics becomes the limit current value of the entire fuel cell stack, so the performance of the lowest unit cell is the fuel cell stack. It greatly affects the overall performance. Therefore, it can be said that the gas diffusion electrode for a fuel cell of the present invention improves the discharge performance of the entire fuel cell stack.
[0041]
In the above embodiment, pure hydrogen is used as the fuel, and air is used as the oxidant gas. However, instead of pure hydrogen, reformed hydrogen containing impurities such as carbon dioxide, nitrogen, carbon monoxide, or the like is used. It is considered that a similar result can be obtained. Further, it is considered that the same result can be obtained even when liquid fuel such as methanol, ethanol, dimethyl ether or the like is used instead of hydrogen. The liquid fuel may be vaporized in advance and then supplied to the fuel electrode.
[0042]
Moreover, in the said Example, although FEP or PTFE was used as a water repellent material, it is thought that the same result is obtained even if it uses another fluororesin or silicone resin. Furthermore, in the said Example, although carbon paper was used as a porous base material, it is thought that the same result is obtained even if it uses carbon cloth, a carbon nonwoven fabric, etc.
[0043]
Further, in the above examples, the catalyst layer and the polymer electrolyte membrane having the above-described configuration are used. However, the present invention is not limited to these, and it is considered that the same result can be obtained when various catalyst layers are used. It is done. In addition, the joined body of the gas diffusion electrode and solid polymer electrolyte of the present invention can be applied to gas generators such as oxygen, ozone and hydrogen, gas purifiers, and various gas sensors such as oxygen sensors and alcohol sensors. Can do.
[0044]
【Effect of the invention】
According to the present invention, by controlling and optimizing the distribution of the water-repellent material in the gas diffusion electrode, excess water is quickly discharged from the gas diffusion electrode to the outside, and the catalyst in the catalyst layer is uniformly distributed. It becomes possible to supply fuel gas and oxidant gas. Therefore, flooding in the gas diffusion electrode can be suppressed, and gas diffusibility and water vapor permeability can be kept good. Furthermore, the effect of suppressing flooding in the gas diffusion electrode can be further improved by making the water diffusion of the gas diffusion electrode smaller than that of the catalyst layer. Moreover, if the gas diffusion electrode of the present invention is used, a fuel cell excellent in discharge performance and durability can be provided.
[Brief description of the drawings]
1 is a schematic cross-sectional view of a gas diffusion electrode for a fuel cell according to the present invention.
[Figure 2] Reference Example 1 is a diagram showing the relationship between the unit cell A, B and C, the battery voltage of the D and the current density of Comparative Examples 1 and 2 and contact the first embodiment.
[3] Reference Example 1 is a diagram showing the relationship of the cell A of Comparative Examples 1 and 2 and Contact Example 1, B and C, the battery voltage of the D and operating time.
[Explanation of symbols]
11 Polymer Electrolyte Membrane 12 Catalyst Layer 13 Gas Diffusion Electrode Base Material 14 Gas Diffusion Electrode 15 MEA
16 Gas flow path 17 Separator

Claims (3)

A gas diffusion electrode comprising a catalyst layer comprising a carbon powder carrying a catalyst and a polymer electrolyte, a porous substrate made of a carbon material and in contact with the catalyst layer, and a water repellent material applied to the porous substrate. There,
The amount of the water repellent material in the porous substrate continuously decreases from the side in contact with the catalyst layer toward the other side;
The catalyst layer includes a water repellent material having a thickness of Wmm and Xg per cm ² ;
When the porous substrate has a thickness of Zmm and contains Yg of water repellent material per cm ² ,
A gas diffusion electrode for fuel cells, wherein (X / W)> (Y / Z) is satisfied. (1) A water-repellent polymer dispersion or a water-repellent polymer solution is applied to one side of a Zmm-thick porous substrate made of a carbon material with Yg of the water-repellent polymer per cm ² as the porous material. Step 1 for coating so as to include a porous base material, and (2) a catalyst having a thickness of Wmm, comprising carbon powder supporting the catalyst and a polymer electrolyte on its surface and containing Xg of a water repellent material per cm ² A process for producing a gas diffusion electrode for a fuel cell, comprising a step 2 of forming a layer so as to satisfy (X / W)> (Y / Z) .   The method for producing a gas diffusion electrode for a fuel cell according to claim 2, further comprising a step of heating the porous substrate to 40 to 180 ° C prior to step 1.

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