JP2022029034A - Separator for fuel cell - Google Patents

Abstract

To provide a separator for a fuel cell having high conductivity and rust resistance.SOLUTION: A separator for a fuel cell according to an embodiment of the present invention includes a metal base material, a first layer containing silver or a silver alloy, placed on the surface of the metal base material, and a second layer containing carbon nanotubes and Si-based binder, arranged on the surface of the first layer, and the first layer has a film thickness of 30 nm or more, the coverage of carbon nanotubes in the second layer is 90 area% or more, and the content of Si-based binder is 40 mass% or more of the total mass of the second layer.SELECTED DRAWING: Figure 1

Description

The present invention relates to a separator for a fuel cell.

Fuel cells obtain electromotive force by electrochemically reacting hydrogen and oxygen. In principle, water is the only product produced by the power generation of fuel cells. Therefore, it is attracting attention as a clean power generation system with almost no load on the global environment.

A fuel cell is configured with a membrane electrode assembly (hereinafter, also referred to as "MEA") in which electrode catalyst layers are arranged on both sides of an electrolyte membrane as a basic unit. During operation of the fuel cell, a fuel gas containing hydrogen is supplied to the electrode catalyst layer on the anode (fuel electrode) side, and an oxidation gas containing oxygen is supplied to the electrode catalyst layer on the cathode (air electrode) side. To get. An oxidation reaction proceeds at the anode and a reduction reaction proceeds at the cathode, supplying electromotive force to an external circuit.

In a fuel cell, usually, a gas diffusion layer is arranged outside each electrode catalyst layer of MEA, and a separator is further arranged outside the gas diffusion layer to form a fuel cell. The fuel cell is usually used as a combination of a plurality of required fuel cell cells (hereinafter, also referred to as "fuel cell stack") based on a desired electric power.

The separator not only cuts off the fuel gas and air supplied to each fuel cell, but also functions as a current transmission unit to the outside. Therefore, the separator is required to have not only airtightness but also conductivity, rust resistance and thermal conductivity. Conventionally, a carbon base material has been used for the separator, but in recent years, a material in which a conductive film is arranged on the surface of the metal base material is also used.

As a member used for the separator for a fuel cell, for example, Patent Document 1 includes a metal substrate and a conductive resin layer formed by electrodeposition so as to cover the metal substrate via an intermediate layer. The intermediate layer is at least one of nickel, palladium, platinum, copper, silver, gold, ruthenium, rubidium, zinc, lead, tin, or an alloy of these metals, and the resin layer contains a conductive material. A characteristic separator for a fuel cell is described. The document describes carbon nanotubes as a conductive material.

Patent Document 2 is a separator for a fuel cell including a metal base material and a surface layer provided on the surface of the metal base material, and the surface layer includes a carbon-based conductive material and a Si-based binder. A fuel cell separator characterized in that the surface coverage of the carbon-based conductive material is 90% or more and the ratio of the Si-based binder is 40% or more in the surface layer is described.

Japanese Unexamined Patent Publication No. 2007-280637 Japanese Unexamined Patent Publication No. 2019-133838

As described above, various members that can be used for fuel cell separators have been developed. However, the conventional separator for a fuel cell or a member that can be used therefor has a problem in terms of conductivity and rust prevention. For example, in the case of the separator described in Patent Document 1, in the environment where the fuel cell is used, the contact portion with the gas diffusion layer is reduced, and the contact resistance in the initial stage and in the corrosive environment may be excessively increased. Therefore, the separator described in the document may have reduced conductivity and rust resistance.

Therefore, it is an object of the present invention to provide a fuel cell separator having high conductivity and rust resistance.

The present inventors have studied various means for solving the above-mentioned problems. The present inventors arrange an intermediate layer composed of a first layer containing silver or a silver alloy on the surface of a metal base material, and further contain a predetermined amount of carbon nanotubes and a Si-based binder on the surface of the intermediate layer. It has been found that by arranging the layers, a separator for a fuel cell with reduced contact resistance with the gas diffusion layer can be obtained even in a corrosive environment. The present inventors have completed the present invention based on the above findings.

That is, the present invention includes the following aspects and embodiments.
(1) With a metal base material
A first layer containing silver or a silver alloy placed on the surface of a metal substrate,
A second layer containing carbon nanotubes and a Si-based binder, which is arranged on the surface of the first layer, is provided.
The first layer has a film thickness of 30 nm or more and has a film thickness of 30 nm or more.
A separator for a fuel cell in which the coverage of carbon nanotubes in the second layer is 90 area% or more, and the content of Si-based binder is 40% by mass or more of the total mass of the second layer.

INDUSTRIAL APPLICABILITY According to the present invention, it becomes possible to provide a separator for a fuel cell having high conductivity and rust resistance.

FIG. 1 is a cross-sectional view of a preferred embodiment of a fuel cell separator according to an aspect of the present invention. FIG. 2 is a graph showing the relationship between the CNT coverage of the separators of Comparative Examples 1 to 9 and the initial contact resistance. In the figure, the horizontal axis is the CNT coverage (area%), and the vertical axis is the initial contact resistance (mΩcm ² ). FIG. 3 is a graph showing the relationship between the Si-based binder content in the film of the second layer in the separators of Comparative Examples 1 to 9 and the contact resistance at the initial stage or after the corrosion resistance test. In the figure, the horizontal axis is the Si-based binder content (mass%) in the film of the second layer, and the vertical axis is the contact resistance (mΩcm) at the initial stage (white square) or after the corrosion resistance test (black painted square). ² ). FIG. 4 is a graph showing the contact resistance of the separators of Comparative Example 4, Comparative Example 11 and Example 1 at the initial stage or after the corrosion resistance test. In the figure, the vertical axis is the contact resistance (mΩcm ² ) at the initial stage (white square) or after the corrosion resistance test (black painted square). FIG. 5 is a graph showing the contact resistance of the separators of Comparative Example 5, Comparative Example 12 and Example 2 at the initial stage or after the corrosion resistance test. In the figure, the vertical axis is the contact resistance (mΩcm ² ) at the initial stage (white square) or after the corrosion resistance test (black painted square). FIG. 6 is a graph showing the contact resistance of the separators of Comparative Example 6, Comparative Example 13 and Example 3 at the initial stage or after the corrosion resistance test. In the figure, the vertical axis is the contact resistance (mΩcm ² ) at the initial stage (white square) or after the corrosion resistance test (black painted square).

Hereinafter, preferred embodiments of the present invention will be described in detail.

<1. Separator for fuel cell>
One aspect of the present invention relates to a separator for a fuel cell. FIG. 1 shows a cross-sectional view of a preferred embodiment of the fuel cell separator according to the present invention. Hereinafter, the fuel cell separator of this embodiment will be described with reference to the drawings.

As shown in FIG. 1, the fuel cell separator 10 of this embodiment has a metal base material 1, a first layer 2 arranged on the surface of the metal base material 1, and a first layer 2 arranged on the surface of the first layer 2. It has two layers and three. The first layer 2 contains silver or a silver alloy. The second layer 3 contains carbon nanotubes (hereinafter, also referred to as “CNT”) 4 and a Si-based binder 5.

In the case of conventional members applied to fuel cell separators, the contact area with the gas diffusion layer is reduced in the fuel cell usage environment, and the contact resistance in the initial and corrosive environments may increase excessively. .. For example, in the case of the separator described in Patent Document 1, if the surface coverage of the conductive material contained in the conductive resin layer is low, the contact portion with GDL is reduced and the initial contact resistance with GDL is increased. there is a possibility. Further, in the case of forming a surface treatment film on the surface of a metal base material in a fuel cell separator, if a resin-based binder is used, the corrosive liquid may permeate in the fuel cell usage environment. As the permeation of the corrosive liquid progresses, an oxide film may grow on the interface between the metal substrate and the surface treatment film, and the contact resistance between the separator and GDL may increase. Further, in the fuel cell separator, when a surface treatment film containing conductive carbon is formed on the surface of the metal base material, the resistance at the interface between the metal base material and the surface treatment film becomes high, and the separator and GDL become Contact resistance between them may increase.

The present inventors arrange an intermediate layer composed of a first layer containing silver or a silver alloy on the surface of a metal base material, and further contain a predetermined amount of carbon nanotubes and a Si-based binder on the surface of the intermediate layer. It has been found that by arranging the layers, a separator for a fuel cell with reduced contact resistance with GDL can be obtained even in a corrosive environment. Therefore, the fuel cell separator of the present embodiment having the above configuration can have high conductivity and rust resistance.

In the separator for a fuel cell of this embodiment, the material constituting the metal base material is preferably stainless steel, a titanium alloy or an aluminum alloy, and more preferably stainless steel. The material is relatively inexpensive and has sufficient strength and processability, so that it is suitable as a base material for a separator.

In the fuel cell separator of this embodiment, the first layer preferably contains silver or a silver alloy, and optionally carbon or nitrogen. When the first layer contains a silver alloy, the metal forming the alloy is preferably gold, copper or tin. As described below, the first layer is preferably formed by a plating process. In this case, the first layer preferably contains silver plating or silver alloy plating. By providing the first layer containing silver or a silver alloy, the conductivity of the fuel cell separator of this embodiment can be improved. For example, in the fuel cell separator of this embodiment, when stainless steel or a titanium alloy is used as the material constituting the metal base material, the contact resistance with the GDL may increase due to the presence of the non-conductor film on the surface layer. There is. Here, by providing the first layer containing silver or a silver alloy, the contact resistance with GDL can be reduced and the conductivity can be improved. Therefore, by providing the first layer having the above-mentioned characteristics, the fuel cell separator of this embodiment can have high conductivity.

The first layer has a film thickness of 30 nm or more. The film thickness of the first layer is preferably in the range of 30 to 500 nm, more preferably in the range of 100 to 200 nm. If the film thickness of the first layer is less than the lower limit, the resistance at the interface between the metal substrate and the second layer containing CNT may increase. Further, when the film thickness of the first layer exceeds the upper limit value, the adhesion to the metal substrate may decrease due to the increase in the film stress. Therefore, by providing the first layer having the film thickness in the above range, the fuel cell separator of this embodiment can have high conductivity.

In the fuel cell separator of this embodiment, the second layer contains a CNT and a Si-based binder. The second layer may preferably contain other components such as a dispersant. The content of other components, for example, the dispersant may be appropriately set based on the contents of the CNT and Si-based binders exemplified below, and is usually in the range of 5 to 40% by mass of the total mass of the second layer. .. By including other components, for example, a dispersant, the CNT and Si-based binders can be uniformly arranged in the second layer. Thereby, the fuel cell separator of this embodiment can have high conductivity.

The Si-based binder contained in the second layer is preferably an inorganic Si-based binder. The content of the Si-based binder is 40% by mass or more, preferably in the range of 40 to 60% by mass, based on the total mass of the second layer. When the content of the Si-based binder is less than the above lower limit value, the corrosive liquid may permeate in the environment where the fuel cell is used. As the permeation of the corrosive liquid progresses, an oxide film may grow on the interface between the metal substrate and the surface treatment film, and the contact resistance between the separator and GDL may increase. Therefore, by providing the second layer containing the Si-based binder having the above-mentioned content, the fuel cell separator of this embodiment can have high conductivity and rust resistance.

In the art, CNT means a carbon material in which the single-walled surface of graphite has a tubular structure. Normally, CNTs with a single-layer tubular structure are referred to as single-walled CNTs, CNTs with a two-layer tubular structure are referred to as double-walled CNTs, and CNTs with three or more layers of tubular structures are referred to as multi-walled CNTs. , Each classify. Normally, in a conductive film containing CNTs, the conductivity can be improved by increasing the number of layers of CNTs. The CNT contained in the second layer may have any of the above-mentioned structures. The average length of CNTs contained in the second layer is preferably in the range of 1 to several tens of μm. If the average length of the CNTs is less than the lower limit, the conductive path may decrease and the conductivity of the second layer may decrease. If the average length of the CNTs exceeds the upper limit, intermolecular aggregation is likely to occur and the CNTs may not be uniformly arranged. Therefore, by providing the second layer containing the CNT having the average length in the above range, the fuel cell separator of this embodiment can have high conductivity and rust resistance.

The content of CNTs contained in the second layer is preferably 20% by mass or more, more preferably 20 to 30% by mass, based on the total mass of the second layer. If the CNT content is less than the lower limit, the contact resistance between the resulting fuel cell separator and GDL may increase. Therefore, the fuel cell separator of this embodiment can have high conductivity by providing the second layer containing the above-mentioned content of CNT.

In the fuel cell separator of this embodiment, the coverage of CNTs in the second layer is 90 area% or more. When the coverage of CNT is less than the lower limit, the initial contact resistance may increase in the measurement of contact resistance described below. Therefore, the fuel cell separator of this embodiment can have high conductivity by providing the second layer containing the CNTs having the coverage in the above range.

In each aspect of the present invention, the coverage of CNTs in the second layer of the fuel cell separator is not limited, but can be measured by the following procedure. The surface of the fuel cell separator is observed using a laser microscope. The observed image is binarized from the viewpoint of the presence or absence of CNT, and the ratio covered by CNT is calculated as the coverage ratio (area%).

The fuel cell separator of this embodiment has an initial contact resistance to GDL of usually 10 mΩcm ² or less, especially 5 mΩcm ² or less. The contact resistance after the corrosion resistance test is usually 5 mΩcm ² or less, especially 4.9 mΩcm ² or less. The fuel cell separator of the present embodiment having the contact resistance in the above range can have high conductivity and rust resistance in the fuel cell usage environment.

In each aspect of the present invention, the contact resistance of the fuel cell separator to GDL can be measured by the following procedure, but is not limited. The film-forming surface of the fuel cell separator and GDL are overlapped with each other, and a predetermined load (for example, 1 MPa) is applied. In that state, a predetermined current (for example, 1 A) is applied and the voltage between the separator and GDL is measured. The measured voltage value is converted into a resistance value, and the evaluation area is further multiplied to calculate the initial contact resistance. The corrosion resistance evaluation can be performed assuming the environment in which the fuel cell is used. First, the fuel cell separator is immersed in a strongly acidic corrosive liquid. With the separator immersed, a potential of a predetermined (for example, 0.9 V) constant voltage is applied between the separator and the electrode. After a lapse of a certain period of time, the contact resistance measured by the same procedure as described above is taken as the contact resistance value after the corrosion resistance test. The strong acid corrosive liquid used is, for example, a strong acid solution containing fluorine (F) and chlorine (Cl) at pH 3.

<2. Manufacturing method of separator for fuel cell>
Another aspect of the present invention relates to a method for manufacturing a separator for a fuel cell according to another aspect of the present invention. The production method of this embodiment includes a metal substrate preparation step, a first layer forming step, and a second layer forming step.

[2-1. Metal substrate preparation process]
This step involves preparing a metal substrate.

The metal base material prepared in this step is preferably composed of the material described above. The metal base material may be prepared by purchasing a product or the like, or may be manufactured and prepared by oneself.

[2-2. First layer forming process]
This step involves forming a first layer containing silver or a silver alloy on the surface of the metal substrate.

This step can be carried out by coating the surface of the metal base material with the material contained in the first layer described above. The first layer can be carried out by, but is not limited to, a film forming means usually used in the art such as plating treatment or physical vapor deposition.

[2-3. Second layer forming process]
This step can be carried out by coating the surface of the first layer with the material contained in the second layer described above. This step involves preparing a composition containing the materials contained in the second layer, applying the composition to the surface of the first layer, and curing the coating film of the applied composition ( Sol-gel method).

The preparation of the composition can be carried out by uniformly dispersing the material having a predetermined content by means commonly used in the art such as a dissolver or a bead mill.

The coating of the composition can be carried out by means commonly used in the art such as coater coating, blade coating, spray spraying or screen printing.

Curing of the coating film can be carried out by heat-treating the coating film after coating at a predetermined temperature. The temperature of the heat treatment may be appropriately set based on the material used.

By carrying out each step described above, a fuel cell separator according to one aspect of the present invention can be obtained.

As described above in detail, the fuel cell separator according to one aspect of the present invention has high conductivity and rust resistance. Therefore, the fuel cell separator according to one aspect of the present invention can be suitably used for a fuel cell of an automobile or the like.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the technical scope of the present invention is not limited to these examples.

<I: Preparation of separator>
First, CNT and a dispersant were added to the Si solution, which is the source of the Si binder, and the mixture was stirred and mixed. At that time, by adjusting the amount of the dispersant, a raw material of a sample having the composition shown in Table 1 was prepared. Next, a first layer containing silver plating was formed on the surface of the metal base material (stainless steel) by a plating treatment. The raw material of each sample was dropped on the surface of the first layer and coated using a bar coater. The coated sample was heated at 300 ° C. for 30 minutes to solidify the coating film, and a separator for each sample was obtained (sol-gel method).

<II: Separator evaluation>
[II-1: Measurement of contact resistance]
The film-forming surface of the separator of each sample was superposed on GDL (TGP-H-060 manufactured by Toray Industries, Inc.), and a load of 1 MPa was applied. In that state, a current of 1 A was applied and the voltage between the separator and GDL was measured. The measured voltage value was converted into a resistance value, and the evaluation area was further multiplied to calculate the initial contact resistance.

The corrosion resistance evaluation was performed assuming the environment in which the fuel cell is used. First, the separator of each prepared sample was immersed in a strong acid corrosive solution. With the separator immersed, a constant voltage potential of 0.9 V was applied between the separator and the electrode. After a lapse of a certain period of time, the contact resistance measured by the same procedure as described above was taken as the contact resistance value after the corrosion resistance test. The strong acid corrosive liquid used is a strong acid solution containing fluorine (F) and chlorine (Cl) at pH 3.

[II-2: Measurement of CNT coverage]
The surface of the separator of each sample was observed using a laser microscope. The observed image was binarized from the viewpoint of the presence or absence of CNT, and the ratio covered by CNT was calculated as the coverage ratio (area%).

<III: Result>
Table 1 shows the composition of the raw materials used to prepare the separator for each sample and the evaluation results of the separator. Figure 2 shows the relationship between the CNT coverage of the separators of Comparative Examples 1 to 9 and the initial contact resistance. In the figure, the horizontal axis is the CNT coverage (area%), and the vertical axis is the initial contact resistance (mΩcm ² ). FIG. 3 shows the relationship between the Si-based binder content in the film of the second layer in the separators of Comparative Examples 1 to 9 and the contact resistance at the initial stage or after the corrosion resistance test. In the figure, the horizontal axis is the Si-based binder content (mass%) in the film of the second layer, and the vertical axis is the contact resistance (mΩcm) at the initial stage (white square) or after the corrosion resistance test (black painted square). ² ). FIG. 4 shows the contact resistance of the separators of Comparative Example 4, Comparative Example 11 and Example 1 at the initial stage or after the corrosion resistance test. In the figure, the vertical axis is the contact resistance (mΩcm ² ) at the initial stage (white square) or after the corrosion resistance test (black painted square). FIG. 5 shows the contact resistance of the separators of Comparative Example 5, Comparative Example 12 and Example 2 at the initial stage or after the corrosion resistance test. In the figure, the vertical axis is the contact resistance (mΩcm ² ) at the initial stage (white square) or after the corrosion resistance test (black painted square). FIG. 6 shows the contact resistance of the separators of Comparative Example 6, Comparative Example 13 and Example 3 at the initial stage or after the corrosion resistance test. In the figure, the vertical axis is the contact resistance (mΩcm ² ) at the initial stage (white square) or after the corrosion resistance test (black painted square).

As shown in Table 1, as the CNT coverage increased, the initial contact resistance decreased. When the CNT coverage was 90 area% or more, the initial contact resistance was 10 mΩcm ² or less (Fig. 2). It is considered that this is because the contact area with GDL increased due to the increase in the CNT coverage. In addition, when the Si-based binder content in the film of the second layer was 40% by mass or more, there was no difference in contact resistance between the initial stage and the corrosion resistance test (Fig. 3). It is considered that the high content of the Si-based binder in the film of the second layer suppressed the permeation of the corrosive liquid and suppressed the growth of the oxide film at the interface between the surface-treated film and the metal substrate. Furthermore, the initial contact resistance is further reduced in the sample in which the Si-based binder content in the second layer film is in the range of 40 to 60% by mass and the film thickness of the first layer (silver plating layer) is 30 nm or more. The contact resistance after corrosion resistance was 5 mΩcm ² or less (Figs. 4 to 6).

Based on the above, in the fuel cell separator, the first layer containing silver or a silver alloy arranged on the surface of the base material and the second layer containing CNT and Si-based binder arranged on the surface of the first layer. The first layer has a film thickness of 30 nm or more, the coverage of CNTs in the second layer is 90 area% or more, and the content of Si-based binder is 40 mass of the total mass of the second layer. By providing a configuration of% or more, the contact resistance with GDL can be reduced to 5 mΩcm ² or less.

The present invention is not limited to the above-described embodiment, and includes various modifications. For example, the above-described embodiment has been described in detail in order to explain the present invention in an easy-to-understand manner, and is not necessarily limited to the one including all the configurations described. In addition, it is possible to add, delete, and / or replace a part of the configuration of each embodiment with another configuration.

1 ... Metal substrate
2 ... First layer
3 ... Second layer
4 ... Carbon nanotubes
5… Si-based binder
10… Separator for fuel cell

Claims (1)

With a metal base material
A first layer containing silver or a silver alloy placed on the surface of a metal substrate,
A second layer containing carbon nanotubes and a Si-based binder, which is arranged on the surface of the first layer, is provided.
The first layer has a film thickness of 30 nm or more and has a film thickness of 30 nm or more.
A separator for a fuel cell in which the coverage of carbon nanotubes in the second layer is 90 area% or more, and the content of Si-based binder is 40% by mass or more of the total mass of the second layer.

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