JP2005149745A - Gas diffusion electrode for fuel cell and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas diffusion electrode for a fuel cell having high conductivity, and to provide its manufacturing method. <P>SOLUTION: This gas diffusion electrode for a fuel cell is characterized by being formed of a non-woven fabric containing a carbon fiber. This manufacturing method of the gas diffusion electrode for a fuel cell is characterized by comprising: a process 1 for preparing diffusion slurry containing a carbon fiber as an essential constituent and containing an organic polymer and/or an inorganic material as an arbitrary constituent; a process 2 for dehydrating the diffusion slurry; and a process 3 for drying the diffusion slurry dehydrated in the process 2. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a gas diffusion electrode for a fuel cell suitably used as an electrode material for a solid polymer electrolyte fuel cell and a method for producing the same.

  A fuel cell is a power generation system that continuously supplies fuel and an oxidant and extracts chemical energy as electric power when they react. Fuel cells are classified into alkaline, phosphoric acid, and solid polymer types that operate at relatively low temperatures, and molten carbonate types and solid oxide electrolyte types that operate at high temperatures, depending on the type of electrolyte used. The

  As shown in FIG. 1, for example, the basic structure of a single cell constituting a solid polymer fuel cell is an electrolyte membrane 12 having ion conductivity and having catalyst layers 11a and 11b disposed on both sides, and an electrolyte membrane. 12 has gas diffusion electrodes 13a and 13b disposed on both outer surfaces of the gas separator 12, and separators 14a and 14b disposed on both surfaces of the gas diffusion electrodes 13a and 13b.

In such a polymer electrolyte fuel cell, first, hydrogen supplied to the separator 14a on the fuel electrode side is guided to the surface of the gas diffusion electrode 13a through the gas flow path in the separator 14a. Next, the hydrogen is uniformly diffused by the gas diffusion electrode 13a, and then introduced to the catalyst layer 11a on the fuel electrode side, where it is separated into hydrogen ions and electrons by a catalyst such as platinum. Then, the hydrogen ions are guided through the electrolyte membrane 12 to the catalyst layer 11b in the oxygen electrode on the opposite side across the electrolyte membrane 12. On the other hand, the electrons generated on the fuel electrode side are guided to the gas diffusion electrode 13b on the oxygen electrode side through the circuit 16 having the load 15, and further to the catalyst layer 11b on the oxygen electrode side. At the same time, oxygen introduced from the separator 14b on the oxygen electrode side passes through the gas diffusion electrode 13b on the oxygen electrode side and reaches the catalyst layer 11b on the oxygen electrode side. Then, water is generated from oxygen, electrons, and hydrogen ions to complete the power generation cycle.
In addition to hydrogen, alcohols such as methanol and ethanol can also be used directly as the fuel used in the polymer electrolyte fuel cell.

In the fuel cell cycle described above, it is required that the electrochemical reaction proceeds continuously on both the fuel electrode side and the oxygen electrode side. For this purpose, gas diffusion capacity, conductivity, heat resistance, chemical resistance, water repellency are required. An excellent gas diffusion electrode is required. In particular, since the conductivity greatly affects the performance of the fuel cell, a highly conductive gas diffusion electrode is required.
As a conventional gas diffusion electrode for a fuel cell, an electrode made of carbon and a PAN-based carbon fiber has been proposed (see, for example, Patent Document 1).
JP 2003-239164 A

However, since the gas diffusion electrode described in Patent Document 1 has insufficient conductivity, electrons generated on the catalyst cannot be efficiently guided to the circuit, and a fuel cell having sufficiently high performance can be obtained. There wasn't.
The present invention has been made to solve the above-described problems, and an object thereof is to provide a gas diffusion electrode for a fuel cell having high conductivity and a method for manufacturing the same.

The gas diffusion electrode for fuel cells of the present invention is characterized by comprising a nonwoven fabric containing carbon nanofibers.
In the gas diffusion electrode for a fuel cell of the present invention, the carbon nanofiber is preferably a carbon nanotube and / or a carbon nanohorn.
The non-woven fabric is either a mixture of carbon nanofibers and organic polymer, a mixture of carbon nanofibers and inorganic material, or a mixture of carbon nanofibers, organic polymer and inorganic material. It is preferable.
At that time, the organic polymer is preferably at least one selected from natural pulp, polyethylene resin pulp, polypropylene resin pulp, and fluororesin, and the inorganic material is at least one selected from glass fiber and stainless fiber. It is preferable that
In the gas diffusion electrode for a fuel cell of the present invention, a catalyst may be supported on the carbon nanofiber.
The gas diffusion electrode for a fuel cell preferably contains catalyst-supported carbon in which a catalyst is supported on carbon other than carbon nanofibers.
Moreover, in the gas diffusion electrode for fuel cells of the present invention, the content of carbon nanofibers is preferably 0.1% by mass or more.
The fuel cell gas diffusion electrode of the present invention preferably has an electric resistance value in the thickness direction of 2.0 mΩ or less. Further, it is preferable that a bulk density of ^{0.05g / cm 3 ~0.50g / cm 3} . Furthermore, it is preferable that thickness is 1.0 micrometer-100 micrometers.
The method for producing a gas diffusion electrode for a fuel cell according to the present invention includes a step 1: preparing a dispersion slurry containing carbon nanofibers as an essential component and an organic polymer and / or an inorganic material as an optional component;
Step 2: dehydrating the dispersed slurry;
Step 3: a step of drying the water dehydrated in Step 2.

The gas diffusion electrode for a fuel cell of the present invention has sufficiently high conductivity and can efficiently conduct electrons generated on the catalyst to a circuit, so that a high-performance fuel cell can be obtained.
According to the method for producing a gas diffusion electrode for a fuel cell of the present invention, carbon nanofibers can be in a highly dispersed state.

The gas diffusion electrode for fuel cells of the present invention (hereinafter abbreviated as “gas diffusion electrode”) is a nonwoven fabric containing carbon nanofibers. This nonwoven fabric has high conductivity because the carbon nanofibers are highly dispersible. Moreover, in many nonwoven fabrics using carbon nanofibers, the carbon nanofiber axial direction is often in the thickness direction, and the electrical resistance value in the thickness direction is low, so the conductivity is high. Therefore, the gas diffusion electrode which consists of a nonwoven fabric containing carbon nanofiber has high electroconductivity. Furthermore, a gas diffusion electrode can be made thin in the nonwoven fabric containing carbon nanofibers. If the gas diffusion electrode is made thinner, the conductivity becomes higher and the fuel cell can be made smaller.
In addition, since the gas diffusion electrode made of a nonwoven fabric containing carbon nanofibers has high surface smoothness, for example, when a membrane electrode assembly (MEA) is manufactured, a pressure as high as several MPa in a state of being joined to a polymer electrolyte membrane. The polymer electrolyte membrane is not easily damaged even if it is applied.

  The nonwoven fabric may be a mixture of carbon nanofibers and organic polymer and / or inorganic material. Here, the organic polymer is preferably at least one selected from fluorine resins such as natural pulp, polyethylene resin (PE) pulp, polypropylene resin (PP) pulp, and polytetrafluoroethylene (PTFE). If an organic polymer is used, the binding between the carbon nanofibers becomes stronger and the strength tends to increase. In particular, when a fluororesin is used, the water repellency of the gas diffusion electrode increases and the power generation characteristics of the fuel cell are further improved. Moreover, as an inorganic material, at least 1 sort (s) chosen from glass fiber and stainless steel fiber is preferable, and stainless steel fiber is especially preferable especially. If stainless steel fiber is used, higher conductivity can be obtained.

  When the nonwoven fabric contains components other than carbon nanofibers, such as a mixture of carbon nanofibers and organic polymer and / or inorganic material, the content of carbon nanofibers is preferably 0.1% by mass or more. , Closer to 100% by mass is more preferable. If the content of the carbon nanofiber is close to 100% by mass, higher conductivity can be obtained. In addition, there exists a possibility that high electroconductivity may not be shown as it is less than 0.1 mass%.

  As the carbon nanofibers constituting the gas diffusion electrode, carbon nanotubes and / or carbon nanohorns are preferably used. Among these, carbon nanotubes are particularly preferable. When carbon nanotubes are used as the electrode material, higher conductivity can be obtained.

In addition, a catalyst may be supported on the carbon nanofiber. If the carbon nanofiber carrying the catalyst is used, the utilization efficiency of the catalyst can be kept high, so that the amount of expensive catalyst can be reduced. In addition, since there is no need to provide a new catalyst layer, there is a great merit in manufacturing.
Moreover, you may make the nonwoven fabric containing carbon nanofiber contain the catalyst carrying | support carbon which carry | supported the catalyst on carbon other than carbon nanofiber. The combined use of the carbon nanofiber carrying the catalyst and the catalyst-carrying carbon is preferable because the utilization efficiency of the catalyst can be further increased.

Here, the catalyst is not particularly limited as long as it can generate hydrogen ions from hydrogen and oxygen ions from oxygen, but it is preferable to mainly use an alloy catalyst such as platinum or platinum and ruthenium. The carbon used for the catalyst-supporting carbon is preferably a high-structure carbon having a large specific surface area and a relatively large size of the secondary agglomerated particles in view of both performance and productivity. Examples of such a carbon black include carbon black typified by furnace black and channel black. Furthermore, among conductive grade carbon blacks, for example, Lion Akzo's Ketchin EC and Cabot's VulcanXC72R are suitable for use in the present invention because of their high dispersibility in slurry and low resistance when used as a catalyst. It is done. However, the carbon black that can be used in the present invention is not limited to these examples, and any grade can be used regardless of the specific surface area or the particle size.
In the catalyst-supporting carbon, carbon particles other than carbon black can be used. As carbon particles other than carbon black, acetylene black, graphite, carbon fiber and the like are suitably used in the same manner as carbon black.
Furthermore, a conductive inorganic material having resistance to an oxidizing atmosphere can be used instead of carbon.

In this gas diffusion electrode, the electrical resistance value in the thickness direction is preferably 2.0 mΩ or less. When the electric resistance value in the thickness direction of the gas diffusion electrode is larger than 2.0 mΩ, the battery performance tends to be deteriorated and the carbon nanofibers tend to become brittle due to heat generation.
The electrical resistance value is a value measured as follows. That is, a fuel cell electrode is sandwiched between two gold-plated 50 mm square (thickness 10 mm) electrodes at a pressure of 1 MPa, and an electrical resistance value (R (mΩ)) between the two electrodes is measured. This value is the electric resistance value in the thickness direction.

The bulk density of the gas diffusion electrode is preferably 0.05 g / cm ^{3 to} 0.50 g / cm ³ . When the bulk density is less than 0.05 g / cm ³ , voids in the electrode increase and the conductive path decreases, so that the internal resistance increases and the electric resistance value tends to increase. When the bulk density exceeds 0.50 g / cm ³ , the gas diffusibility tends to decrease.
The bulk density is a value obtained from the following calculation formula (1).
Formula (1):
Bulk density (g / cm ³ ) = mass (g) / {unit area of electrode (cm ² ) × thickness (cm)}

The thickness of the gas diffusion electrode is preferably 1.0 μm to 100 μm. When the thickness is less than 1.0 μm, there is a tendency that the strength is lowered and it is easy to cut during processing. When the thickness exceeds 100 μm, the electrical resistance value in the thickness direction increases, and there is a tendency for problems such as difficulty in downsizing the battery.
In addition, the thickness of the gas diffusion electrode is a value measured using a micrometer (dot type thickness meter).

Next, the manufacturing method of the gas diffusion electrode for fuel cells of this invention is demonstrated.
In the method for producing a gas diffusion electrode for a fuel cell of the present invention, a dispersion slurry containing carbon nanofibers as an essential component and an organic polymer and / or an inorganic material as an optional component is prepared (step 1) and dehydrated (step 1). Step 2) is a method of drying (step 3) to form a sheet (nonwoven fabric).
In such a production method, a wet papermaking method performed by ordinary papermaking can be applied. In the wet papermaking method, first, a dispersion slurry is prepared by stirring and mixing a predetermined amount of a carbon nanofiber simple substance slurry and a mixed slurry containing an organic polymer and / or an inorganic material as necessary. When stirring in water, an ultrasonic generator may be used.
Next, the concentration of the dispersed slurry is adjusted, and this slurry is supplied to a wet papermaking machine such as a long net type or a circular net type, and dehydrated by a dewatering part on a continuous wire mesh. Then, the dehydrated one is dried to obtain a gas diffusion electrode made of nonwoven fabric.
The gas diffusion electrode obtained by such a manufacturing method has an excellent feature of high conductivity because the carbon nanofibers are in a highly dispersed state.

Hereinafter, the present invention will be described in more detail based on examples and comparative examples. However, the present invention is not limited to these examples.
<Example 1: Carbon nanotube (CNT) only>
In a container containing water, 100 g of carbon nanotubes were put as a raw material, and while stirring with a stirrer while generating ultrasonic waves with an ultrasonic generator, they were uniformly dispersed to prepare a dispersed slurry. A predetermined amount of this dispersed slurry was collected, and a wet paper was prepared using a standard hand-making apparatus defined in JIS P8222: 1998. Thereafter, press dehydration was performed, and wet paper was dried using a Yankee dryer whose temperature was adjusted to 130 ° C. to obtain a gas diffusion electrode.

<Example 2: CNT / fluororesin (PTFE) = 99/1>
A gas diffusion electrode was obtained in the same manner as in Example 1 except that 99 g of carbon nanotubes and 1 g of PTFE were used as raw materials.
<Example 3: CNT / polypropylene resin (PP) = 99/1>
A gas diffusion electrode was obtained in the same manner as in Example 1 except that 99 g of carbon nanotubes and 1 g of PP pulp were used as raw materials.
<Example 4: CNT / polyethylene resin (PE) = 99/1>
A gas diffusion electrode was obtained in the same manner as in Example 1 except that 99 g of carbon nanotubes and 1 g of PE pulp were used as raw materials.
<Example 5: CNT / natural pulp (NP) = 99/1>
A gas diffusion electrode was obtained in the same manner as in Example 1 except that 99 g of carbon nanotubes and 1 g of natural pulp were used as raw materials.

<Example 6: CNT / PP / NP = 80/10/10>
A gas diffusion electrode was obtained in the same manner as in Example 1 except that 80 g of carbon nanotubes, 10 g of PP pulp, and 10 g of natural pulp were used as raw materials.
<Example 7: CNT / PE / NP = 80/10/10>
A gas diffusion electrode was obtained in the same manner as in Example 1 except that 80 g of carbon nanotubes, 10 g of PE pulp, and 10 g of natural pulp were used as raw materials.
<Example 8: CNT / PP / PE = 80/10/10>
A gas diffusion electrode was obtained in the same manner as in Example 1 except that 80 g of carbon nanotubes, 10 g of PP pulp, and 10 g of PE pulp were used as raw materials.
<Example 9: CNT / stainless fiber = 99/1>
A gas diffusion electrode was obtained in the same manner as in Example 1 except that 99 g of carbon nanotubes and 1 g of stainless fiber were used as raw materials.
<Example 10: CNT / PP / stainless fiber = 80/10/10>
A gas diffusion electrode was obtained in the same manner as in Example 1 except that 80 g of carbon nanotubes, 10 g of PP pulp, and 10 g of stainless fiber were used as raw materials.

<Example 11: CNT / PP = 50/50>
A gas diffusion electrode was obtained in the same manner as in Example 1 except that 50 g of carbon nanotubes and 50 g of PP pulp were used as raw materials.
<Example 12: CNT / PP = 10/90>
A gas diffusion electrode for a fuel cell was obtained in the same manner as in Example 1 except that 10 g of carbon nanotubes and 90 g of PP pulp were used as raw materials.
<Example 13>
A gas diffusion electrode was obtained in the same manner as in Example 1 except that 100 g of carbon nanotubes carrying a platinum catalyst was used as a raw material.
<Example 14>
A gas diffusion electrode was obtained in the same manner as in Example 1, except that 90 g of carbon nanotubes and 10 g of carbon black carrying a platinum catalyst (platinum supported catalyst TEC10E50E) were used as raw materials.
<Comparative Examples 1 and 2>
A carbon cloth manufactured by E-TEK was used as a gas diffusion electrode. In addition, this carbon cloth is produced from carbon fibers that are not carbon nanofibers.

<Evaluation results of electrode properties and fuel cell power generation characteristics>
Table 1 shows the evaluation results of the electrode physical properties of the gas diffusion electrodes of Examples 1 to 14 and Comparative Examples 1 and 2 and the power generation characteristics of the fuel cell equipped with the physical properties.

In addition, the fuel cell used for evaluation was produced as follows.
In Examples 1 to 12 and Comparative Example 1, first, two 25 mm square gas diffusion electrodes were prepared for the fuel electrode and the oxygen electrode, respectively. In the gas diffusion electrode for the fuel electrode, a catalyst slurry made of carbon, a carbon nanotube and an organic solvent carrying platinum catalyst and ruthenium catalyst, and the amount of carbon carrying platinum catalyst and ruthenium catalyst is 0.00. Coating was carried out to 3 mg / cm ² . The gas diffusion electrode for the oxygen electrode is coated with a catalyst slurry composed of carbon carrying a platinum catalyst and an organic solvent so that the amount of carbon carrying the platinum catalyst is 0.3 mg / cm ^2. Worked. Next, two gas diffusion electrodes coated with a catalyst slurry are placed in contact with both surfaces of the ion exchange membrane (Dafon Nafion 117), and bonded by a hot press (120 ° C.). And an ion exchange membrane (MEA). The MEA obtained above was incorporated into a single cell to obtain a cell for evaluation.
In Examples 13 and 14, the catalyst slurry was not applied to the gas diffusion electrode and used as it was.
In Comparative Example 2, a slurry in which carbon nanotubes were mixed and dispersed was used as the fuel electrode catalyst slurry and the oxygen electrode catalyst slurry. That is, in the gas diffusion electrode for the fuel electrode, a catalyst slurry composed of carbon, carbon nanotubes, and organic solvent carrying platinum catalyst and ruthenium catalyst, and the amount of carbon carrying platinum catalyst and ruthenium catalyst is 0. The coating was performed so that the amount of carbon nanotubes was 3 mg / cm ² and 0.05 mg / cm ² . Further, in the gas diffusion electrode for the oxygen electrode, a catalyst slurry composed of carbon carrying a platinum catalyst, carbon nanotubes, and an organic solvent is used, and the amount of carbon carrying a platinum catalyst is 0.3 mg / cm ² . Coating was performed so that the amount of the nanotubes was 0.05 mg / cm ² . Other than that, it was the same as Comparative Example 1.

Hydrogen and oxygen were used as the gas supplied to each cell of the fuel cell. All the supply gases were humidified by bubbling to a supply pressure of 2.5 atm, and held at a temperature of 70 ° C. for the single cell. Table 1 shows the results of examining the voltage at a current density of 1 A / cm ² under these conditions.

As shown in Table 1, in Examples 1 to 14, the gas diffusion electrode is made of a nonwoven fabric containing carbon nanofibers, and the gas diffusion electrode itself has high conductivity, and the thickness can be reduced and the bulk density is increased. Since it was in an appropriate range, the conductivity was further increased (electric resistance value of 2.0 mΩ or less). And the fuel cell provided with this gas diffusion electrode had a high voltage at a current density of 1 A / cm ² and was excellent in power generation performance.
In Comparative Examples 1 and 2, the gas diffusion electrode was a carbon fiber electrode that did not contain a conventional carbon nanofiber, and the power generation performance of the fuel cell was low.

Next, using the gas diffusion electrodes of Example 1, Comparative Example 1 and Comparative Example 2, a single cell similar to the above was assembled, and a diluted methanol solution (5% by mass of methanol) was evaluated as fuel. . Table 2 shows the results of examining the voltage at a current density of 1 A / cm ² .

From the results in Table 2, when a gas diffusion electrode made of a nonwoven fabric containing carbon nanofibers is used, the voltage at a current density of 1 A / cm ² is high, and good performance in a methanol liquid polymer electrolyte fuel cell. It was confirmed that

It is sectional drawing which shows the basic structure of the single cell which comprises a polymer electrolyte fuel cell.

Claims (14)

  A gas diffusion electrode for fuel cells, comprising a nonwoven fabric containing carbon nanofibers.   The gas diffusion electrode for a fuel cell according to claim 1, wherein the carbon nanofiber is a carbon nanotube and / or a carbon nanohorn.   The gas diffusion electrode for a fuel cell according to claim 2, wherein the nonwoven fabric is a mixture of carbon nanofiber and organic polymer.   The gas diffusion electrode for a fuel cell according to claim 2, wherein the nonwoven fabric is a mixture of carbon nanofibers and an inorganic material.   The gas diffusion electrode for a fuel cell according to claim 2, wherein the nonwoven fabric is a mixture of carbon nanofiber, organic polymer, and inorganic material.   6. The gas diffusion electrode for a fuel cell according to claim 3, wherein the organic polymer is at least one selected from natural pulp, polyethylene resin pulp, polypropylene resin pulp, and fluororesin.   The gas diffusion electrode for a fuel cell according to claim 4 or 5, wherein the inorganic material is at least one selected from glass fiber and stainless fiber.   The gas diffusion electrode for a fuel cell according to any one of claims 1 to 7, wherein a catalyst is supported on the carbon nanofiber.   The gas diffusion electrode for a fuel cell according to any one of claims 1 to 8, wherein a catalyst-supporting carbon in which a catalyst is supported on carbon other than carbon nanofibers is included.   The gas diffusion electrode for a fuel cell according to any one of claims 1 to 9, wherein the content of the carbon nanofiber is 0.1% by mass or more.   The gas diffusion electrode for a fuel cell according to any one of claims 1 to 10, wherein an electric resistance value in a thickness direction is 2.0 mΩ or less. The gas diffusion electrode for a fuel cell according to any one of claims 1 to 11, wherein the bulk density is 0.05 g / cm ^{3 to} 0.50 g / cm ³ .   The gas diffusion electrode for a fuel cell according to any one of claims 1 to 12, wherein the thickness is 1.0 µm to 100 µm. Step 1: preparing a dispersion slurry containing carbon nanofibers as an essential component and an organic polymer and / or inorganic material as an optional component;
Step 2: dehydrating the dispersed slurry;
Step 3: A method for producing a gas diffusion electrode for a fuel cell, comprising: drying the material dehydrated in Step 2.

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