JP2023079940A - Fuel cell separator titanium material, method for manufacturing the same, fuel cell separator and fuel cell - Google Patents

Abstract

To provide a fuel cell separator titanium material used for a fuel cell separator, capable of reducing contact resistance between separators and suppressing increase in the contact resistance in cooling water environment.SOLUTION: A fuel cell separator titanium material 1 includes: a titanium foil base material 2 having a first surface (one surface 2a) orthogonal to the thickness direction and a second surface (the other surface 2b); and a conductive maintenance coating 5 formed on at least the first surface 2a of the titanium foil base material 2. The conductive maintenance coating 5 includes a TiC layer 3; the TiC layer 3 directly contacts the titanium foil base material 2; and at least a part of the TiC layer 3 is exposed to the surface of the conductive maintenance coating 5.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a titanium material for fuel cell separators capable of maintaining high conductivity for a long period of time and a manufacturing method thereof, and further to a fuel cell separator using the titanium material for fuel cell separators, and a fuel cell.

A plurality of battery cells constituting a fuel cell are coated with a catalyst such as platinum on both sides of a solid polymer membrane, and a pair of carbon papers are arranged on both sides of the polymer membrane to diffuse the fuel gas uniformly on the catalyst surface. A pair of separators are arranged on both sides of the . A fuel cell is configured by stacking a plurality of cells. Separators are press-formed with channels for oxygen and hydrogen gas, which are fuels for power generation. The separators of adjacent battery cells are in direct contact with each other, and the battery cells are electrically connected in series. High voltage can be obtained.

As the base material of the separator, stainless steel and titanium are generally used from the viewpoint of press moldability and corrosion resistance. These metals have a passivation film with low conductivity formed on the surface, which maintains corrosion resistance.

However, on the electrode side of the separator, the SO ₃ ⁻ ions in the solid polymer membrane are eluted and sulfuric acid is produced, creating an acidic corrosive environment. Therefore, if the stainless steel material or titanium material is used as it is, it is difficult to achieve both corrosion resistance and electrical conductivity. Therefore, conventionally, the surface of stainless steel materials and titanium materials is coated with films such as precious metal films such as gold and platinum that can prevent corrosion and maintain conductivity, carbon films, and nitride films and carbide films that have high conductivity. is used. However, since the noble metal film is expensive, there is a problem that the raw material cost of the separator increases. In addition, since the carbon film has poor adhesion to the substrate, it is necessary to form an intermediate layer or the like to improve the adhesion, which complicates the manufacturing process of the separator.

Furthermore, when the surface of a stainless steel material or titanium material is coated with a highly conductive film such as a nitride film or a carbide film, high conductivity can be obtained even in an acidic environment on the electrode side. Corrosion resistance is inferior to carbon film. Therefore, when exposed to the environment inside the fuel cell (electrode side), it corrodes and the resistance gradually increases.

Since TiC films, which are representative of carbonized films, also have high conductivity, various studies have been conducted as conductive coating films for fuel cell separators. For example, Patent Literature 1 proposes a metal separator for a fuel cell in which a thin film containing TiC or the like is formed on all or part of the surface or periphery of a metal member that contacts an electrode.
In addition, in Patent Document 1, a thin film is formed by spraying fine powder together with a carrier gas from the tip of a microporous nozzle on all or part of a portion of the surface of a metal member that is in contact with an electrode. A method for producing a fuel cell metallic separator is also described, in which the temperature of the metallic member is normal temperature to 200.degree.

However, according to Non-Patent Document 1, although the initial contact resistance can be greatly reduced by forming a titanium-based conductive material such as TiC, it is exposed to an acidic environment (in a sulfuric acid aqueous solution) simulating the internal environment of a fuel cell. It is described that the contact resistance is remarkably increased due to discoloration when exposed to heat.

Therefore, Patent Document 2 describes a fuel cell separator whose surface is covered with a carbon layer and in which an intermediate layer made of titanium carbide is formed at the interface between the carbon layer and the substrate. In Patent Document 2, after a carbon layer is formed on the surface of a base material by a vapor deposition method, the base material on which the carbon layer is formed is heat-treated to obtain a titanium layer between the base material and the carbon layer. A method of forming an intermediate layer made of carbide (TiC) has been proposed. According to the above method, since the TiC intermediate layer is formed by the reaction of carbon and titanium, it is possible to improve the adhesion between each layer such as a conductive thin film that affects conductivity and corrosion resistance and the base material, and the conductive durability of the separator. can improve sexuality.

As described above, in the fuel cell separator, various proposals have been conventionally made for the environment inside the fuel cell (electrode side).

JP 2005-268081 A JP 2012-43775 A

"Tetsu to Hagane" vol. 104, No. 5 (2018), p. 264-273

Fuel cells generate electricity by reacting hydrogen and oxygen to synthesize water, which also generates heat. The cell is cooled by running cooling water through the In such a fuel cell, when the surfaces of the separators corrode in an environment in which cooling water flows, the contact resistance between the separators increases, which may cause power generation loss. Moreover, if the contact resistance becomes even higher, it may become impossible to generate electricity.

Therefore, if the stainless steel material or the titanium material is used as it is, the contact resistance between the separators increases, resulting in a large power generation loss. Therefore, the surface of the separator on the cooling water flow path side is required to have the performance of maintaining corrosion resistance and electrical conductivity even in an environment where it is exposed to the cooling water.

However, in conventional titanium materials for fuel cell separators, a surface coating that is highly resistant to acidic environments and capable of maintaining electrical conductivity has been studied for the surface on the inner side (electrode side) of the fuel cell. The surface film on the cooling water side has not been sufficiently studied. That is, at least on the inner side of the fuel cell, a highly corrosion-resistant and highly conductive surface film is formed, and on the cooling water side, the passive film remains or the same film as that on the inner side of the fuel cell is formed. There are many things.

In addition, when the TiC intermediate layer and the carbon layer on the surface described in Patent Document 2 are formed on both the fuel cell inner side and the cooling water side of the titanium base material, the decrease in conductivity can be suppressed. can be done. However, the TiC intermediate layer and the carbon layer are preferably formed after removing the passivation film on the surface of the base material, and the process of removing the passivation film takes time, so there is a problem that productivity decreases. be.

The present invention has been made in view of such problems. Among the base materials used for fuel cell separators, the titanium material, which has particularly high corrosion resistance, can reduce the contact resistance between the separators. An object of the present invention is to provide a titanium material for a fuel cell separator that can suppress an increase in contact resistance in a cooling water environment.
Another object of the present invention is to provide a method for producing a titanium material for a fuel cell separator, which can extremely easily produce a titanium material for a fuel cell separator that has high corrosion resistance and can suppress an increase in contact resistance. and
A further object of the present invention is to provide a fuel cell separator comprising the titanium material for a fuel cell separator and a fuel cell comprising the fuel cell separator.

The above object of the present invention is achieved by the following configuration [1] relating to a titanium material for a fuel cell separator.

[1] A titanium foil substrate having a first surface and a second surface orthogonal to a thickness direction, and a conductive retention coating formed on at least the first surface of the titanium foil substrate,
The conductivity preserving coating includes a TiC layer, the TiC layer being in direct contact with the titanium foil base material, and at least a portion of the TiC layer being exposed on the surface of the conductivity preserving coating. A titanium material for a fuel cell separator, characterized by:

Further, preferred embodiments of the present invention relating to titanium materials for fuel cell separators relate to the following [2] to [6].

[2] The electroconductivity retaining film has a carbon region part on a part of the surface opposite to the surface where the TiC layer and the titanium foil base are in contact, and the TiC layer and the carbon region The titanium material for a fuel cell separator according to [1], wherein the electroconductivity-retaining film is constituted by a portion.

[3] The titanium material for a fuel cell separator according to [1] or [2], wherein the conductive retaining film is formed only on the first surface of the titanium foil base material.

[4] The titanium material for a fuel cell separator according to [3], characterized in that a functional coating different from the conductive retaining coating is formed on the second surface of the titanium foil base material. .

[5] The titanium material for a fuel cell separator according to any one of [1] to [4], wherein the film thickness of the electroconductive retaining film is 5 nm or more and 50 nm or less.

[6] Used in fuel cell separators included in battery cells,
The titanium material for a fuel cell separator according to any one of [1] to [5], characterized in that the electrically conductive retaining film is arranged on the outer surface side of the battery cell.

Further, the above object of the present invention is achieved by the following configuration [7] relating to a method for producing a titanium material for a fuel cell separator.

[7] A carbon film forming step of forming a carbon film by physical vapor deposition on one surface of a titanium foil substrate having a passivation film formed on the surface;
an annealing treatment step of heating the titanium foil base on which the carbon film is formed in a vacuum or in an inert gas atmosphere;
In the annealing treatment step, after oxygen in the passivation film is diffused and absorbed in the titanium foil base material, carbon in the carbon film reacts with titanium in the titanium foil base material, and the titanium foil is A method for producing a titanium material for a fuel cell separator, characterized in that a conductive retaining coating is formed having a TiC layer in contact with one surface of a substrate.

Further, preferred embodiments of the present invention relating to a method for producing a titanium material for fuel cell separators relate to the following [8] to [12].

[8] The method for producing a titanium material for a fuel cell separator according to [7], wherein in the carbon film forming step, the carbon film is formed with a thickness of 5 nm or more and 30 nm or less.

[9] The Fe content in the titanium foil base material before the carbon film forming step is 0.15% by mass or less, and the O content is 0.15% by mass or less, A method for producing a titanium material for a fuel cell separator according to [7] or [8].

[10] The method for producing a titanium material for a fuel cell separator according to any one of [7] to [9], wherein the annealing temperature in the annealing step is 700°C or higher and 850°C or lower. .

[11] The titanium for a fuel cell separator according to any one of [7] to [10], wherein the carbon film is formed by an arc ion plating method in the carbon film forming step. How the material is made.

[12] The method according to any one of [7] to [11], further comprising a molding step of press-molding the titanium foil base material on which the conductive retention film is formed after the annealing step. 2. A method for producing a titanium material for a fuel cell separator according to 1.

Further, the above object of the present invention is achieved by the following configuration [13] relating to a fuel cell separator.

[13] A fuel cell separator included in a battery cell,
provided so as to sandwich an electrolyte membrane having electrode layers formed on both sides and a pair of gas diffusion layers provided on both sides of the electrolyte membrane from the outer surface side,
Having the titanium material for a fuel cell separator according to any one of [1] to [5],
A fuel cell separator, wherein the second surface of the titanium foil substrate is arranged to face the gas diffusion layer.

Further, the above object of the present invention is achieved by the following configuration [14] relating to a fuel cell.

[14] A fuel cell comprising a plurality of stacked battery cells,
Each battery cell has a fuel cell separator according to [13] on its outer surface,
Adjacent fuel cell separators between the adjacent battery cells are in contact with each other at least partially, and cooling water flow paths are formed between the adjacent fuel cell separators,
A fuel cell characterized in that the conductive retaining film of the titanium material for fuel cell separators is present on the surfaces of the adjacent fuel cell separators facing each other.

According to the present invention, in the titanium foil base material used for the fuel cell separator, the contact resistance between the separators can be reduced, and the increase in the contact resistance can be suppressed in a cooling water environment. A titanium material and its manufacturing method, a fuel cell separator, and a fuel cell can be provided.

FIG. 1 is a schematic diagram showing the structure of a titanium material for a fuel cell separator according to this embodiment. FIG. 2A is a schematic diagram showing a method of measuring the contact resistance of a TiC sintered body. FIG. 2B is a top view showing a contact portion of two TiC sintered bodies. FIG. 3 is a graph showing changes in contact resistance, with the vertical axis representing the contact resistance and the horizontal axis representing the number of days of immersion. FIG. 4A is a cross-sectional view showing a method for manufacturing a titanium material for a fuel cell separator according to an embodiment of the present invention in order of steps. FIG. 4B is a diagram showing a method for manufacturing a titanium material for a fuel cell separator according to an embodiment of the present invention in order of steps, and is a cross-sectional view showing the next step of FIG. 4A. FIG. 4C is a diagram showing a method for manufacturing a titanium material for a fuel cell separator according to an embodiment of the present invention in order of steps, and is a cross-sectional view showing the next step of FIG. 4B. FIG. 4D is a diagram showing a method for manufacturing a titanium material for a fuel cell separator according to an embodiment of the present invention in order of steps, and is a cross-sectional view showing the next step of FIG. 4C. FIG. 5A is a schematic diagram showing an example of an AIP device. FIG. 5B is a side view showing a portion of FIG. 5A. FIG. 6 is a schematic diagram showing a battery cell using the fuel cell separator according to the embodiment of the invention. FIG. 7 is a schematic diagram showing a fuel cell having a fuel cell separator according to an embodiment of the invention. FIG. 8A is a drawing-substituting photograph showing a cross-sectional TEM image after the formation of the carbon film. FIG. 8B is a drawing-substituting photograph showing an EDX image of oxygen in FIG. 8A. FIG. 8C is a drawing-substituting photograph showing an EDX image of titanium in FIG. 8A. FIG. 9A is a drawing-substituting photograph showing a cross-sectional TEM image of the test piece after annealing. FIG. 9B is a drawing-substituting photograph showing an EDX image of oxygen in FIG. 9A. FIG. 9C is a drawing-substituting photograph showing an EDX image of titanium in FIG. 9A. FIG. 10 is a drawing-substituting photograph showing an electronic analysis figure of the test piece after annealing. FIG. 11A is a schematic diagram showing a method of measuring contact resistance of a press-molded simulated test piece. FIG. 11B is a top view showing contact portions of two press-molded simulated test pieces.

EMBODIMENT OF THE INVENTION Hereinafter, the form (henceforth "this embodiment") for implementing this invention is demonstrated in detail. It should be noted that the present invention is not limited to the embodiments described below, and can be arbitrarily modified without departing from the gist of the present invention.
Further, in this specification, the term "to" indicating a numerical range is used to include the numerical values before and after it as lower and upper limits.

[Titanium material for fuel cell separators]
In order to obtain a titanium material for a fuel cell separator that can suppress an increase in contact resistance in a cooling water environment, the present inventors conducted extensive studies and found that in an acidic environment such as the inside of a fuel cell, contact It was found that a TiC layer, which was difficult to use due to its increased resistance, can maintain a low contact resistance in a cooling water environment. The titanium material for a fuel cell separator according to this embodiment will be described in detail below. In this specification, the titanium material for fuel cell separators may be simply referred to as titanium material.

<Structure of Titanium Material for Fuel Cell Separator>
FIG. 1 is a schematic diagram showing the structure of a titanium material for a fuel cell separator according to this embodiment.
The titanium material 1 includes a titanium foil base material 2 and a conductive maintenance film 5 formed on one surface (first surface) 2a of the titanium foil base material 2 perpendicular to the thickness direction. In the present embodiment, the other surface (second surface) 2b of the titanium foil substrate 2 perpendicular to the thickness direction is not formed with the conductive retention film 5. A film or the like is present, but illustration of the passive film on the other surface 2b is omitted.

The conductive retention coating 5 is composed of a TiC layer 3 and a carbon region portion 4 , and the TiC layer 3 is in direct contact with the titanium foil substrate 2 . In addition, the conductive retention film 5 has a carbon region portion 4 on its outermost surface, that is, part of the surface opposite to the surface where the TiC layer 3 and the titanium foil base material 2 are in contact. However, the carbon region portion 4 does not cover the entire surface of the titanium material 1 , and at least a portion of the TiC layer 3 is exposed on the surface of the conductivity retaining film 5 .

In the fuel cell separator titanium material 1 configured in this way, the TiC layer 3 is exposed on at least a part of the surface, and even when the TiC layer 3 is exposed to the cooling water environment for a long time, A desired contact resistance can be maintained. The properties of TiC are described in detail below.

The inventors conducted the following experiments to confirm that TiC can maintain low contact resistance even in a cooling water environment. FIG. 2A is a schematic diagram showing a contact resistance measuring method, and FIG. 2B is a top view showing a contact portion of two TiC sintered bodies.
The contact resistance of the TiC sintered body was measured as follows.
First, two plate-shaped TiC sintered bodies 64 and 66 having a length of 4 cm, a width of 2 cm, and a thickness of 2 mm were prepared. Since a TiC sintered body has a front side and a back side, the front side and the back side of the two TiC sintered bodies 64 and 66 are set in advance. Next, these two TiC sintered bodies 64 and 66 were immersed in neutral cooling water for fuel cells at 80° C. for 42 days (about 1000 hours). After that, the TiC sintered body 64 and the TiC sintered body 66 were overlapped so as to form a cross so that the front surfaces thereof were in contact with each other, thereby producing a test material 67 .

As shown in FIG. 2A, the contact resistance measuring instrument 30 sandwiches a test material 67 between a pair of cylindrical copper electrodes 31a and 31b, and between one copper electrode 31a and the other copper electrode 31b, A four-terminal method resistance measuring instrument 33 is connected via the current terminals 32a and 32b. A load applying device (not shown) is attached to the copper electrodes 31a and 31b so that a load can be applied to the test material 67 in the direction indicated by the arrow.

Then, the test material 67 was inserted into the contact resistance measuring instrument 30 so that the load of the copper electrode 31a and the copper electrode 31b was applied to the overlapping portion of the TiC sintered body 64 and the TiC sintered body 66. . After that, a load of 40 kg (surface pressure: 10 kg/cm ² ) was applied to the test material 67 by the copper electrodes 31a and 31b. After that, one current terminal 32a of the four-terminal method resistance measuring instrument 33 is connected to the copper electrode 31a, the other current terminal 32b is connected to the copper electrode 31b, and one resistance measurement terminal 38a is connected to one TiC sintered body 64. , the other resistance measuring terminal 38b was connected to the other TiC sintered body 66 to measure the resistance. After that, the contact resistance was calculated by multiplying the obtained resistance value by the contact area of 4 cm ² .
In the same way, the TiC sintered body 64 and the TiC sintered body 66 are overlapped in a cross shape so that their back surfaces are in contact with each other to prepare a test material, and the contact resistance is calculated. bottom.

FIG. 3 is a graph showing changes in contact resistance, with the vertical axis representing the contact resistance and the horizontal axis representing the number of days of immersion. Using the contact resistance measuring device 30 shown in FIGS. 2A and 2B, 4 days, 10 days, 21 days, and 42 days after immersion, the test material with the front surfaces in contact with each other and the back surfaces were in contact with each other. As a result of measuring the contact resistance of the test materials, all the test materials hardly changed from the initial value and maintained a constant value of about 1.6 mΩ·cm ² . From these results, it can be shown that it is preferable to form a TiC layer on the cooling water side.

In this embodiment, the conductivity preserving film 5 is composed of the TiC layer 3 and the carbon region portion 4, but the conductivity preserving film 5 may consist of the TiC layer 3 only. Whether or not the carbon region portion 4 is present changes depending on the conditions in the manufacturing method to be described later. As shown by the contact resistance test results, the TiC layer 3 has the effect of preventing an increase in contact resistance even when exposed to a cooling water environment. Further, since the carbon region portion 4 also has conductivity and corrosion resistance, high resistance to cooling water can be obtained even in a state where the carbon region portion 4 exists on a part of the surface of the conductive film 5 . However, since the carbon region 4 has lower conductivity than the TiC layer 3, the presence of the carbon region 4 on the entire surface of the conductivity retaining film 5 makes it possible to obtain the desired conductivity. Can not. Therefore, in the present invention, at least a portion of the TiC layer is exposed on the surface of the conductive retaining film 5.

As described above, the conductive retaining film 5 in this embodiment is formed on the cooling water side of the fuel cell separator titanium material 1, that is, at least on one surface 2a of the titanium foil substrate 2. As shown in FIG. Since the other surface of the titanium foil substrate 2 needs to have corrosion resistance and electrical conductivity in an acidic environment, the conductive retention coating 5 is provided only on one surface (first surface) 2a of the titanium foil substrate 2. is preferably formed. In addition, on the other surface (second surface) 2b of the titanium foil base material 2, a functional film having excellent corrosion resistance and electrical conductivity in an acidic environment, which is different from the conductive film 5, is formed. is more preferred. As the functional coating, for example, it is preferable to use a mixed coating of carbon black and titanium oxide as described in JP-A-2018-170291. After press molding, after removing the passivation film on the surface, an amorphous carbon film or a Ti—Nb composite oxide film may be formed and used as the functional film. The reason why the amorphous carbon film and the Ti—Nb composite oxide film are formed after press molding is that these films have poor adhesion to the substrate and are peeled off during press molding.

(Film thickness of the electroconductivity retention film: 5 nm or more and 50 nm or less)
In the fuel cell separator titanium material 1 according to the present embodiment, the electroconductivity retaining film 5 is formed by forming a carbon film on the passive film on the surface of the titanium foil base material 2 and annealing it under desired conditions. can be obtained by A method for producing a titanium material for a fuel cell separator will be described later, but in order to obtain a TiC layer 3 with a desired thickness, a carbon film formed by stress caused by a difference in thermal expansion from the titanium base material during annealing treatment is used. In order to suppress the peeling of the carbon film, the thickness of the carbon film to be formed is preferably 5 nm or more and 30 nm. When the carbon film has the above range, the carbon film becomes a TiC layer by annealing, and the layer thickness increases due to the density difference of the film. Therefore, it is preferable that the film thickness of the electroconductivity-retaining film to be obtained is 5 nm or more and 50 nm or less. In this embodiment, in the field of view when the cross section of the sample is observed with a magnification of 500,000 times with a TEM, the thickest part, the thinnest part, and the intermediate thickness part The thickness was measured at three points, and their arithmetic mean was defined as the film thickness.

[Method for producing titanium material for fuel cell separator]
The present inventors formed a carbon film on the surface of the passive film of titanium without removing the passive film, and performed annealing treatment under predetermined conditions to burn off the passive film and cool the cooling water. It was found that a TiC layer having excellent resistance to was formed. The method for producing the titanium material for fuel cell separators will be described in more detail below.

As mentioned above, TiC has a high resistance to cooling water. Accordingly, the present inventors conducted various studies on methods for forming a TiC layer on the surface of a titanium foil base material. Methods for forming a TiC layer include (1) heating a titanium foil substrate in a hydrocarbon gas to react titanium with the hydrocarbon gas to form a TiC layer; and (2) chemical vapor deposition. (CVD: Chemical Vapor Deposition) method, (3) physical vapor deposition (PVD: Physical Vapor Deposition) method, and the like. Since (1) and (2) generate TiC in gas, a TiC layer is formed on the entire surface including one side and the other side of the titanium foil base material. When a TiC layer is formed on the battery-side surface, if the formed TiC layer is used as it is, it will corrode in an acidic environment and the contact resistance will increase.

Further, when a carbon film or the like is formed on the TiC layer, corrosion resistance to an acidic environment can be ensured, but if pinholes exist in the carbon film, corrosion progresses from the pinholes and the resistance increases. Therefore, it is necessary to form the carbon film thick enough to prevent the formation of pinholes, and it takes time to form the film.

Furthermore, there is a method of forming a TiC layer on the surface of a titanium foil base material by using the PVD method of (3), for example, by a sputtering method or an ion plating method. In order to form a film with high adhesion, it is necessary to remove the passive film on the surface in advance by Ar ion bombardment in a vacuum chamber. However, titanium has the property of easily taking in oxygen, as it is called an oxygen getter. Therefore, even if the passivation film on the surface of the titanium foil base material is removed by Ar ion bombardment, the passivation film is removed by absorbing the oxygen of the moisture that is adsorbed to the inner surface of the vacuum chamber and slowly released. A new passivation film immediately regenerates from the damaged area, so it takes time to completely remove it.

In addition, the above Patent Document 1 describes a method of forming a TiC thin film by spraying fine powder together with a carrier gas from the tip of a microporous nozzle on all or part of the portion of the surface of the metal member that comes into contact with the electrode. It is However, when the metal member is made of titanium, a passivation film is present, so there is a possibility that sufficient adhesion between the TiC thin film and the titanium base material cannot be obtained. Therefore, if the TiC thin film is formed by the above method prior to press molding for forming flow paths for cooling water, oxygen gas, and hydrogen gas, the TiC thin film cannot follow the plastic deformation of the base material due to press molding. There is a risk of peeling.

Therefore, in the present embodiment, a carbon film is formed on the passive film of the titanium foil base material, and the carbon film is heated in a vacuum or in an inert gas atmosphere to eliminate the passive film, thereby removing the titanium foil. A TiC layer is formed in direct contact with the substrate. Therefore, it is possible to prevent an increase in the contact resistance of the surface on the cooling water side and easily form a TiC layer having excellent adhesion to the titanium foil substrate.

A method for producing a titanium material for fuel cell separators according to an embodiment of the present invention will be described below with reference to the drawings.

As shown in FIG. 4A, a titanium foil substrate 2 is prepared. A passive film (amorphous titanium oxide layer) 6 is originally present on the surfaces (one surface 2 a and the other surface 2 b ) of the titanium foil base material 2 . 4A to 4D, the illustration of the other surface of the titanium foil base material 2 is omitted.

Next, as shown in FIG. 4B, a carbon film 7 is formed by physical vapor deposition on one surface 2a of the titanium foil substrate 2 having the passive film 6 (carbon film forming step).

After that, as shown in FIG. 4C, the titanium foil base material 2 with the carbon film 7 formed thereon is heated in a vacuum or in an inert gas atmosphere (annealing treatment step). In this annealing process, oxygen in the passivation film 6 is diffused and absorbed into the titanium foil base material 2, and the passivation film 6 disappears.

Thereafter, as shown in FIG. 4D, carbon in the carbon film 7 and titanium in the titanium foil base material 2 react with each other in an annealing treatment step to form a TiC layer in direct contact with one surface 2a of the titanium foil base material 2. 3 is formed. Depending on the annealing conditions, carbon that has not reacted with titanium may remain as a carbon region 4 on a portion of the outermost surface of the titanium foil substrate 2 on the one surface 2a side. However, when all the carbon in the carbon film 7 reacted with the titanium in the titanium foil base material 2 in the annealing process, only the TiC layer 3 was formed on the one surface 2a of the titanium foil base material 2. structure.

In this specification, the film including the TiC layer 3 is referred to as the conductive preserving film 5 . That is, the conductivity retaining film 5 may be composed of only the TiC layer 3 or may be composed of the TiC layer 3 and the carbon region portion 4, but at least part of the TiC layer 3 is conductive. It is assumed that it is exposed on the surface of the property retaining film 5 .

After that, although illustration is omitted, it is preferable to have a molding step of press-molding the titanium foil base material 2 having the conductive retention film 5 formed on the surface thereof. After that, it is more preferable to further form a functional coating having a material or structure different from that of the conductive retaining coating 5 and having excellent corrosion resistance in an acidic environment on the other surface 2b of the titanium foil base material 2. .

As described above, according to the method for producing a titanium material for a fuel cell separator according to the present embodiment, the annealing treatment (heat treatment) is performed after the carbon film 7 is formed, so that the mechanical properties necessary for press molding can be obtained. Moreover, according to the above manufacturing method, it is possible to easily form a TiC layer capable of preventing an increase in contact resistance on the cooling water side surface. Furthermore, in the present embodiment, since the TiC layer 3 is formed by the reaction between carbon and titanium, the adhesion between the TiC layer 3 and the titanium foil base material 2 is improved, and after forming the conductivity maintaining film 5, pressing is performed. Even if it is molded, it does not come off and can maintain conductivity on the cooling water side. In addition, when the conductive maintenance film 5 is formed only on one surface 2a of the titanium foil base material 2 that contacts the cooling water, the other surface 2b is provided with functionality that exhibits corrosion resistance in the environment inside the battery. A film can be separately formed, and conductivity can be maintained on the inner surface side of the battery as well.

Furthermore, in the method for producing a titanium material for fuel cell separators according to the present embodiment, conditions of each step will be described in detail.

<Carbon film deposition process>
(Titanium foil base material)
In this embodiment, the titanium foil base material 2 on which the carbon film 7 is formed is preferably made of pure titanium. By using the titanium foil base material 2 made of pure titanium, the press moldability can be improved as compared with the case of using a titanium foil base material made of a titanium alloy. Even if the titanium foil base material 2 is made of pure titanium, if there are many impurities, the press formability after annealing is deteriorated, and the β phase, which deteriorates the press formability, is precipitated in the annealing process. easier. Therefore, it is preferable that the amount of impurities in the titanium foil base material 2 is small.

Preferred contents of components contained in the titanium foil base material 2 are described below.
The contents of the impurity components contained in the titanium foil base material 2 are Fe: 0.15 mass % or less, O: 0.15 mass % or less, C: 0.08 mass % with respect to the total mass of the titanium foil base material. % or less, and N: 0.03 mass % or less. In addition, in particular, Fe and O are elements that easily precipitate the β phase in the annealing process, so it is more preferable that Fe: 0.1 mass% or less and O: 0.1 mass% or less, Fe: 0 08% by mass or less, and O: 0.08% by mass or less is more preferable.

(Thickness of titanium foil base: 0.05 mm or more and 0.3 mm or less)
If the thickness of the titanium foil base material 2 is less than 0.05 mm, the base material becomes too thin and may break during press molding. Therefore, the titanium foil base material 2 preferably has an appropriate thickness. . On the other hand, as the thickness of the titanium foil substrate increases, the width of fine flow paths for hydrogen and oxygen gas becomes narrower, possibly making it difficult for the gas to flow. In addition, when the thickness of the titanium foil substrate exceeds 0.3 mm, it is necessary to increase the size of the fuel cell separator in order to secure the desired flow path width. Connect. Therefore, the thickness of the titanium foil substrate is preferably 0.05 mm or more and 0.3 mm or less, and more preferably 0.08 mm or more and 0.15 mm or less.

(Method of forming carbon film)
In this embodiment, the carbon film 7 is formed by a physical vapor deposition (PVD) method. The PVD method is a method of forming a desired film on the surface of a substrate by a sputtering method, an arc ion plating method (hereinafter sometimes referred to as an "AIP method"), or the like. According to the PVD method, a carbon film (DLC film: diamond-like carbon film) 7 can be formed on only one side of the titanium foil substrate 2 by evaporating carbon using a solid carbon target such as graphite. In particular, use of the AIP method is preferable because the deposition rate of the carbon film 7 is high and the conductivity of the obtained carbon film 7 is high when the AIP method is used.

An apparatus for forming the carbon film 7 only on one surface 2a of the titanium foil base material 2 using the AIP method will be briefly described below. FIG. 5A is a schematic diagram showing an example of an AIP device, and FIG. 5B is a side view showing part of FIG. 5A.
As shown in FIGS. 5A and 5B, a circular rotary table 42 is rotatably arranged inside the AIP chamber 40 . In addition, four supports 43 are provided at substantially equal intervals near the periphery of the rotary table 42, and the titanium foil base material 2 faces one surface 2a outward of the supports 43 in the radial direction. configured to be retained. Furthermore, a carbon target 41 is arranged on the inner side surface of the AIP chamber 40, and a linear line extending from the upper surface to the lower surface is provided in the vicinity of the surface opposite to the side surface where the carbon target 41 is installed. A tungsten filament 44 is installed.

When forming the carbon film 7 using such an AIP apparatus, first, after introducing Ar into the AIP chamber 40, a current is applied to the tungsten filament 44 while the turntable 42 is being rotated. red-hot. Next, a bias voltage is applied between the tungsten filament 44 and the titanium foil substrate 2 so that the titanium foil substrate 2 side becomes negative. As a result, Ar ions collide with the surface of the titanium foil substrate 2 to remove stains on one surface 2a of the titanium foil substrate 2 by sputtering.
After that, while rotating the rotary table 42, an arc discharge is generated between the carbon target 41 and the AIP chamber 40 to emit carbon atoms, so that only one surface 2a of the titanium foil substrate 2 is exposed. A carbon film 7 can be formed on the substrate.

When the AIP method is used, the ionization rate of the emitted carbon atoms is high, so applying a negative bias voltage to the titanium foil substrate 2 promotes densification of the obtained carbon film 7 . However, in general, the film stress becomes the highest when a bias voltage of about 100 V is applied, and when a bias voltage higher than that is applied, the temperature rises and the film stress decreases. Therefore, in order to suppress peeling of the carbon film 7 due to increased film stress, the bias voltage applied to the titanium foil substrate 2 during the formation of the carbon film 7 should be 50 V or less, or 150 V or more. preferably.

In addition, it is preferable that hydrogen and oxygen, which are impurities, in the formed carbon film 7 be as small as possible. If the content of hydrogen or oxygen in the carbon film 7 is large, the electrical conductivity of the carbon region 4 becomes low when the carbon region 4 remains on the surface of the conductivity retaining film 5 in the annealing process. As a result, the contact resistance between the separators increases, which may result in power generation loss. When the carbon film (DLC film) 7 is formed by a PVD method such as a sputtering method or an AIP method, the source of hydrogen and oxygen taken into the carbon film 7 may be moisture adsorbed on the inner surface of the chamber. many. Therefore, it is preferable to remove the moisture adsorbed on the inner surface of the chamber by, for example, heating the chamber before forming the carbon film 7 .

(Thickness of carbon film: 5 nm or more and 30 nm)
If the film thickness of the carbon film 2 formed in the carbon film forming step is less than 5 nm, the carbon in the TiC layer 3 is reduced in the subsequent annealing treatment step by further heating after the TiC layer 3 is formed. may be diffused and absorbed in the titanium foil base material 2 and the TiC layer 3 may not remain. On the other hand, since the carbon film (DLC film) 7 originally has poor adhesion to the titanium foil base material 2, if the film thickness of the carbon film 2 to be formed exceeds 30 nm, it will not adhere to the titanium base material during the annealing treatment. The carbon film 2 may peel off due to the stress caused by the difference in thermal expansion, which may make it difficult to form a uniform TiC layer 3 . Therefore, the film thickness of the carbon film 7 formed by the carbon film forming process is preferably 5 nm or more and 30 nm or less, and more preferably 10 nm or more and 25 nm or less. In this embodiment, in the field of view when the cross section of the sample is observed with a magnification of 500,000 times with a TEM, the thickest part, the thinnest part, and the intermediate thickness part The thickness was measured at three points, and their arithmetic mean was defined as the film thickness.

<Annealing process>
After forming the carbon film 2 on the passive film 6 on the surface of the titanium foil base material 2, when this is heated in a vacuum or in an inert gas atmosphere, oxygen in the passive film 6 is first converted into titanium foil. It diffuses and absorbs into the base material 2 . The reaction between the titanium in the titanium foil substrate 2 and the carbon in the carbon film 2 is initiated after the oxygen in the passive film 6 is diffused and absorbed into the titanium foil substrate 2 and the passive film 6 disappears. be. The passivation film 6 usually has an amorphous structure, and the Ti/O composition ratio and thickness are not always uniform. As a result, the timing at which the carbon in the carbon film 2 and the titanium in the titanium foil substrate 2 contact, that is, the timing at which the reaction starts differs depending on the location. Therefore, since the reaction time between carbon and titanium becomes uneven depending on the location, the thickness of the TiC layer 3 formed by the annealing process is not necessarily uniform, and the partially unreacted carbon region 4 may remain in the form of islands on the surface of the conductive retaining film 5 . However, since the unreacted carbon region 4 also has conductivity and corrosion resistance, although the conductivity is lower than that of the TiC layer 3, the carbon region 4 is present on a part of the surface of the conductivity retaining film 5. can maintain the desired electrical conductivity even when exposed to cooling water.

In the present embodiment, if the area of the unreacted carbon region portion 4 is 50% or less of the total surface area of the conductivity-preserving film 5, the conductivity against the cooling water environment can be further maintained. The area of the unreacted carbon region portion 4 is preferably 40% or less, more preferably 30% or less, of the total surface area of the conductive maintenance coating 5 .

As described above, since it is preferable to reduce hydrogen and oxygen impurities in the carbon film 7, the annealing is performed in vacuum or in an inert gas atmosphere such as Ar. In addition, in order to prevent the carbon film 7 from being oxidized to CO ₂ and to prevent the other surface 2b of the titanium foil base material 2 on which the carbon film 7 is not formed from being oxidized, It is preferable to suppress the oxygen partial pressure of the atmosphere in the annealing process. Specifically, it is preferable to set the oxygen partial pressure to 0.1 Pa or less.

(Annealing temperature: 700°C or higher and 850°C or lower)
If the annealing temperature in the annealing step is less than 700°C, it may be difficult to obtain the desired mechanical properties of the titanium foil base material necessary for press molding. In addition, the carbon in the carbon film 7 may not sufficiently react with the titanium in the titanium foil base material 2 to form a TiC layer having a desired thickness.
On the other hand, the annealing temperature is not specified because it is also affected by the impurity components and their contents in the titanium foil base material 2. The mechanical properties of the substrate 2 may be impaired. Therefore, the annealing temperature in the annealing treatment step is preferably 700° C. or higher and 850° C. or lower, more preferably 750° C. or higher and 820° C. or lower.

(annealing time: 20 seconds or more and 60 seconds or less)
Since the setting of the annealing time is affected by the annealing temperature, it is not particularly specified in the present embodiment, but it is preferably 20 seconds or more and 60 seconds or less. When the annealing temperature is low, it is preferable to lengthen the annealing time, and when the annealing temperature is high, it is preferable to shorten the annealing time. By adjusting in this way, it is possible to obtain a TiC layer having mechanical properties necessary for press molding and excellent resistance to cooling water.

Here, the annealing time is the time from when the titanium foil base material 2 with the carbon film 7 formed on the surface is put into the heating furnace to when it is taken out. Therefore, since the time required for the temperature of the titanium foil base material 2 to rise from room temperature to the furnace temperature is also included, it is preferable to adjust the annealing time according to the material of the heater of the furnace. For example, the emissivity of the heater also changes the time it takes to reach the set temperature. It is preferable to appropriately adjust the annealing temperature and the annealing time according to the material of the heater.

[Fuel cell separator]
FIG. 6 is a schematic diagram showing a battery cell using the fuel cell separator according to the embodiment of the invention. As shown in FIG. 6, a platinum catalyst (electrode layer) 52 is arranged on both surfaces perpendicular to the thickness direction of a solid polymer membrane (electrolyte membrane) 51, and a pair of carbon paper (gas diffusion layer) is provided on both surfaces. ) 53 are arranged. The fuel cell separator 10 according to the present embodiment is arranged on the outermost surface of these to form the battery cell 50 . The pair of fuel cell separators are connected to a motor or the like via a lead wire (not shown).

In the battery cell 50 configured as described above, when the H ₂ gas is supplied to the channel groove formed between one fuel cell separator 10 and the carbon paper 53 , the H ₂ gas passes through the carbon paper 53 . diffuses and reaches the platinum catalyst 52 . Here, the H ₂ gas becomes hydrogen ions H ⁺ by the platinum catalyst 52 and reaches the platinum catalyst 52 after permeating through the solid polymer membrane 51 . O ₂ gas is supplied to the channel groove formed between the other fuel cell separator 10 and the carbon paper 53, and H ₂ O is synthesized by the platinum catalyst 52 and discharged. In this way, due to the movement of hydrogen ions, the electrons e ⁻ move in the direction opposite to that of the hydrogen ions, so that a current is generated and a motor or the like can be driven.

The fuel cell separator 10 according to this embodiment has the titanium material 1 for a fuel cell separator according to this embodiment described above, and constitutes flow paths for H ₂ gas and O ₂ gas and flow paths for cooling water, which will be described later. Therefore, it is press molded. Then, the fuel cell separator 10 is arranged so that the conductive retaining film is on the outer surface side of the battery cell 50 .
Note that SO ₃ ⁻ in the solid polymer is eluted between the pair of fuel cell separators 10 to produce sulfuric acid, creating an acidic corrosive environment. Therefore, it is preferable that the titanium material 1 for a fuel cell separator according to the present embodiment has a functional film having high corrosion resistance in an acidic environment formed on the other surface 2b of the titanium foil base material 2, for example. As a result, the surface having the conductive retention film is arranged on the cooling water side so as to be in contact with the cooling water, and the function maintenance film is arranged on the inside of the battery so as to be exposed to an acidic environment.

By arranging the fuel cell separator 10 according to this embodiment in this way, the conductive retention film 5 is arranged on the outer surface of the battery cell 50 . Therefore, when the battery cells 50 are stacked and cooling water is flowed between the battery cells 50 as will be described later, high conductivity can be maintained for a long period of time due to the presence of the conductivity retaining film 5 .

[Fuel cell]
FIG. 7 is a schematic diagram showing a fuel cell having a fuel cell separator according to an embodiment of the invention. In FIG. 7, the same or equivalent parts as those shown in FIG. 6 are denoted by the same reference numerals, and the description thereof will be omitted or simplified.
The fuel cell according to this embodiment includes a plurality of stacked battery cells 50 described above. Each battery cell 50 has the fuel cell separator 10 according to this embodiment on its outer surface. Adjacent fuel cell separators 10 in adjacent battery cells 50 are in contact with each other at least partially, and cooling water flow paths are formed between adjacent fuel cell separators 10 .
As described above, the fuel cell separator 10 has the fuel cell separator titanium material 1 according to the embodiment of the present invention. A conductive retaining coating 5 of the battery separator titanium material 1 is present.

In the fuel cell configured in this manner, the conductive retaining film 5, which has high resistance to the cooling water and can prevent a decrease in conductivity, is present on the side of the cooling water flow path. It can be maintained for a period of time, and excellent battery performance can be obtained.

Hereinafter, examples and comparative examples of the titanium material for fuel cell separators according to the present embodiment will be described.
Test pieces of titanium materials for fuel cell separators were prepared by various manufacturing methods described below, and their structures were observed and their contact resistances were measured.

[Preparation of test piece]
<Preparation of titanium foil substrate>
A titanium foil having a thickness of 0.1 mm was cut into a size of 10 cm×20 cm to obtain a titanium foil base material. As the titanium foil, the standard values for the chemical components were Fe: 0.10 mass%, O: 0.08 mass%, C: 0.03 mass%, and N: 0.03 mass%. .

<Carbon film deposition process>
Each post 43 at the periphery of a rotary table 42 having a diameter of 20 cm and placed in the AIP chamber 40 shown in FIGS. , the inside of the AIP chamber 40 was evacuated. Next, while rotating the rotary table 42 at a speed of 3 rpm, Ar gas was introduced into the AIP chamber 40 to adjust the pressure in the chamber to 2 Pa. After that, a current of 300 V is applied between the tungsten filament 44 and the titanium foil base material 2 so that the titanium foil base material 2 side becomes minus. A bias voltage was applied for 40 seconds. Ar ions thus generated were caused to collide with the surface of the titanium foil substrate 2 to remove stains on the surface of the titanium foil substrate 2 by sputtering.

After that, the rotational speed of the rotary table 42 is set to 5 rpm, the Ar pressure in the AIP chamber 40 is set to 0.5 Pa, and an arc discharge is generated between the carbon target 41 and the AIP chamber 40 with a discharge current of 40 A. was allowed to evaporate the carbon. Thus, a carbon film (DLC film) 7 was formed on one surface 2 a of the titanium foil base material 2 . The passivation film on the surface of the titanium foil substrate 2 was not removed. No bias voltage was applied to the titanium foil substrate 2 during the formation of the carbon film 7 (the film was formed at a ground potential of 0 V). In this way, test specimens were produced with three types of carbon film 7 deposition times of 12 seconds, 24 seconds, and 36 seconds.

(Measurement of film thickness)
The film thickness and elemental analysis of the carbon film 7 were carried out by cross-sectional TEM-EDX (Transmission Electron Microscopy-Energy Dispersive X-ray Spectroscopy) analysis on the test piece on which the carbon film 7 was formed in a film formation time of 36 seconds.
TEM-EDX analysis was performed as follows. In order to protect the outermost surface of the test piece, an osmium film is formed with an osmium coater, a carbon film is formed with a high vacuum deposition device, and a tungsten film is formed with a focused ion beam (FIB). After forming a film, a small piece was excised by FIB microsampling. After that, the excised piece was thinned by FIB processing to a thickness that allows TEM observation, and TEM-EDX observation was performed.

FIG. 8A is a drawing-substituting photograph showing a cross-sectional TEM image after the formation of the carbon film. 8B is a drawing substitute photograph showing an EDX image of oxygen in FIG. 8A, and FIG. 8C is a drawing substitute photograph showing an EDX image of titanium in FIG. 8A. As shown in FIG. 8A, the film thickness of the carbon film 7 was substantially uniform and was 30 nm. Since the thickness of the carbon film 7 is considered to be proportional to the film formation time, the film thickness of the carbon film 7 of the test piece whose film formation time is 12 seconds and the test piece whose film formation time is 24 seconds is , are estimated to be 10 nm and 20 nm, respectively. The thickness of the film was measured at three points in FIG. 8A taken at a magnification of 500,000 times: the thickest part, the thinnest part, and the intermediate thickness. and the arithmetic mean of them was taken as the value.

(Observation of structure)
As shown by the oxygen image in FIG. 8B and the titanium image in FIG. 8C, the surface of the titanium foil base material 2 has a passivation film 6 made of oxygen and titanium with a high oxygen concentration and having a thickness of 5 to 10 nm. It was confirmed that It was also confirmed that the carbon film 7 was present between the passive film 6 and the osmium film 8 . Furthermore, as shown in the TEM image of FIG. 8A, it was also confirmed that the carbon film 7 was in close contact with the passivation film 6 without any gaps.

<Annealing process>
Annealing treatment was performed using a vacuum heating apparatus consisting of two chambers, a heating chamber and a sample chamber, capable of transferring samples between the sample chamber and the heating chamber. First, a titanium foil base material on which a carbon film 7 was formed in a film forming time of 36 seconds was placed on a tray in a sample chamber. Next, after the inside of the chamber was evacuated to 0.01 Pa or less, the heater in the heating chamber was heated to 760° C., and Ar gas was introduced to 40 Pa. Thereafter, the titanium foil base material is transported from the sample chamber to the heating chamber, heated for 40 seconds, then transported again to the sample chamber to complete the annealing. The material was taken out and used as a test piece after annealing.

Note that there is a temperature difference between the heater set temperature and the temperature reached by the titanium foil. For example, the heater temperature was set to 615°C, the titanium foil base material welded with a thermocouple was inserted into the heating chamber, and the ultimate temperature of the titanium foil base material was measured, which was about 630°C. From this, it is considered that the temperature of the titanium foil substrate is 15° C. higher than the heater set temperature in the furnace used for annealing. Therefore, when the heater temperature is set to 760°C, it is estimated that the temperature of the titanium foil base material is about 775°C.

(Evaluation of TiC layer)
Cross-sectional TEM-EDX analysis and electron beam diffraction were performed on the annealed test piece. FIG. 9A is a drawing-substituting photograph showing a cross-sectional TEM image of the test piece after annealing. 9B is a drawing-substituting photograph showing an EDX image of oxygen in FIG. 9A, and FIG. 9C is a drawing-substituting photograph showing an EDX image of titanium in FIG. 9A. Moreover, FIG. 10 is a drawing substitute photograph which shows the electronic analysis figure of the test piece after annealing.

From the electron beam diffraction image shown in FIG. 10, it was analyzed that a cubic crystal structure was formed and the interplanar spacing of (220) was 0.153 nm. In addition, the lattice constant was calculated to be 0.433 nm from the following formula (1), which coincided with the lattice constant of TiC, confirming that TiC was formed.

As shown in the cross-sectional TEM image of FIG. 9A, the oxygen image of FIG. 9B, and the titanium image of FIG. , and a carbon region portion 4 made only of carbon. Furthermore, it was confirmed that the passive film 6 that had existed before the annealing process disappeared. The TiC layer 3 was formed with a non-uniform thickness of about 15 to 40 nm, and unreacted carbon regions 4 remained on the outermost surface here and there. Further, the thickness of the conductivity retaining film composed of the TiC layer 3 and the carbon region portion 4 was approximately 30 to 40 nm.

Although the film thickness of the TiC layer 3 was not confirmed for the test piece in which the film formation time of the carbon film 7 was 12 seconds and the test piece in which the film formation time was 24 seconds, the film thickness of the TiC layer 3 was equal to that of the carbon film 7. It is considered to be approximately proportional to the thickness. Therefore, the TiC layer 3 whose deposition time is 12 seconds for the carbon film 7 has a thickness of 1/3 of the TiC layer whose deposition time is 36 seconds, and the TiC layer 3 whose deposition time is 24 seconds is , the thickness of the TiC layer is considered to be ⅔ of the thickness of the 36 sec TiC layer. That is, the thickness of the conductive holding film composed of the remaining carbon region portion 4 and the TiC layer 3 is about 10 to 13 nm in the case of film formation for 12 seconds, and about 20 to 27 nm in the case of film formation for 24 seconds. Conceivable. From these facts, when the film formation time of the carbon film 7 is set to about 11 seconds, or when the film thickness of the carbon film 7 is set to 9 nm, almost all of the carbon film 7 becomes the TiC layer 3. it is conceivable that.

<Press molding simulation process>
First, the titanium foil base material after the annealing treatment step was cut so that the length in the rolling direction was 65 mm and the length in the direction perpendicular to the rolling direction was 20 mm. Next, a center line was drawn at the center of the direction (longitudinal direction) of 65 mm, and two marked lines were drawn with a marker parallel to the center line at positions 12.5 mm apart from the center line toward both end faces. That is, the width between the two marked lines was set to 25.0 mm. After that, a press-molding simulated test piece that simulates press-molding by holding both ends in the longitudinal direction with a tensile tester and pulling it so that the width between the marked lines is 32.5 mm (elongation rate: 30%). was made. In addition, in order to measure the contact resistance between the conductive holding films, two press-molded simulated test specimens were produced from titanium foil substrates treated under the same conditions.

(Measurement of initial contact resistance)
FIG. 11A is a schematic diagram showing a method of measuring the contact resistance of press-molded simulated test pieces, and FIG. 11B is a top view showing contact portions of two press-molded simulated test pieces. In FIG. 11A, the same parts as those in FIG. 2A are denoted by the same reference numerals, and detailed description thereof will be omitted.
The initial contact resistance was measured using the contact resistance measuring device 30 shown in FIG. 11A. Specifically, a resin sheet 35 having a thickness of 0.05 mm and having a hole of 16 mm in diameter was formed near the center of the surface of the press-molded test piece 34 prepared as described above on which the conductive retention film was formed. were placed one on top of the other so that the hole was covered with the press-molded simulated test piece 34 . After that, on the resin sheet 35, another press-molded simulated test piece 36 was placed so that the surface on which the conductive retention film was formed was on the resin sheet 35 side, and the holes in the resin sheet 35 were hidden. Placed on top of each other. In this manner, a test material 37 was prepared so that the conductive holding films of the two press-molded simulated test pieces 34 and 36 were in contact with each other through the holes 35 a of the resin sheet 35 .

After that, in the area of the hole 35a of the resin sheet 35, a load is applied by a pair of cylindrical copper electrodes 31a and 31b having a diameter of 14 mm and an area of 1.54 cm ² (that is, the resin sheet is The three-ply test material 37 was inserted so as not to be sandwiched between the electrodes. After that, a load of 15.4 kg (surface pressure: 10 kg/cm ² ) was applied, and the test material 37 was pressed between the copper electrodes 31a and 31b. After that, one current terminal 32a of a four-terminal method resistance measuring device (manufactured by Tsuruga Electric: low resistance meter 356E) 33 is connected to the copper electrode 31a, and the other current terminal 32b is connected to the copper electrode 31b. 38a and 38b were connected to the ends of the two test pieces, respectively, and the resistance was measured. After that, the initial contact resistance before immersion was calculated by multiplying the obtained resistance value by the contact area of 1.54 cm ² .

As an example, when measuring the contact resistance on the side of the conductive retention film, the contact resistance was measured so that the surfaces on which the conductive retention film was formed were in contact with each other. The contact resistance of titanium was also measured by bringing the back sides (titanium surfaces) on which no property-retaining film was formed into contact with each other to obtain a comparative example.

(Measurement of contact resistance after immersion in cooling water)
The change in contact resistance after exposure to cooling water was investigated. Specifically, cooling water for a fuel cell vehicle (80 to 90 mass % ethylene glycol content) was placed in a polyethylene container, and the test piece whose initial resistance was measured was immersed in the cooling water and sealed. Then, after holding for 4 days (96 hours) in a constant temperature furnace at 80 ° C., the container is removed from the constant temperature furnace, the test piece is removed from the cooling water, washed with ion-exchanged water and dried. Contact resistance was measured in the same way.

(Evaluation result of contact resistance)
The initial contact resistance and 4 The contact resistance after immersion in cooling water for days is shown in Table 1 below. The "*" in the columns of the thickness of the carbon film and the conductivity-preserving film in Table 1 below indicates the thickness of the carbon film and the conductivity-preserving film when the carbon film was formed for 36 seconds. It is a value calculated in proportion to the film formation time from the film thickness of . In this example, a contact resistance of 3 mΩ·cm ² or less was considered acceptable.

As shown in Table 1 above, the film thickness of the conductive retention film was estimated to be 10 to 40 nm in all test pieces in which the film thickness of the carbon film was estimated to be 10 nm to 30 nm. In addition, in the examples in which the conductive retention films were brought into contact with each other, the initial resistance was as low as 0.3 mΩ·cm ² , and the contact resistance after immersion in cooling water for 4 days was 1.1 mΩ·cm at the highest. ² , and all of them were 3 mΩ·cm ² or less.
On the other hand, in the comparative examples in which the titanium surfaces on which the conductive retention film was not formed were brought into contact with each other, the initial resistance was 3 mΩ·cm ² or less, but the contact resistance after immersion in cooling water for 4 days was , both exceeded 3 mΩ·cm ² , and it was confirmed that the passive film on the surface of the titanium foil base material further oxidized and grew in the cooling water, resulting in a large increase in the contact resistance. The titanium surface, that is, the other surface 2b of the titanium foil base material 2 is arranged so as to face the inside of the battery cell. It is preferable to form a functional film that can prevent an increase in resistance.

1 Titanium Material for Fuel Cell Separator 2 Titanium Foil Base Material 3 TiC Layer 4 Carbon Region Part 5 Conductive Retaining Film 6 Passive Film 7 Carbon Film 10 Fuel Cell Separator 50 Battery Cell 51 Solid Polymer Film 52 Platinum Catalyst

Claims (14)

A titanium foil substrate having a first surface and a second surface orthogonal to a thickness direction, and a conductive retention coating formed on at least the first surface of the titanium foil substrate,
The conductivity preserving coating includes a TiC layer, the TiC layer being in direct contact with the titanium foil base material, and at least a portion of the TiC layer being exposed on the surface of the conductivity preserving coating. A titanium material for a fuel cell separator, characterized by: The conductive coating has a carbon region part on a part of the surface opposite to the surface where the TiC layer and the titanium foil base material are in contact, and the TiC layer and the carbon region part 2. The titanium material for a fuel cell separator according to claim 1, wherein said electrically conductive retaining coating is formed. 3. The titanium material for a fuel cell separator according to claim 1, wherein said conductive retention coating is formed only on the first surface of said titanium foil substrate. 4. The titanium material for a fuel cell separator according to claim 3, wherein the second surface of the titanium foil substrate is provided with a functional coating different from the conductive retaining coating. The titanium material for a fuel cell separator according to any one of claims 1 to 4, characterized in that the film thickness of said conductive retaining film is 5 nm or more and 50 nm or less. Used in fuel cell separators contained in battery cells,
The titanium material for a fuel cell separator according to any one of claims 1 to 5, characterized in that said conductive retaining film is arranged on the outer surface side of said battery cell. a carbon film forming step of forming a carbon film by physical vapor deposition on one surface of a titanium foil base material having a passivation film formed on the surface;
an annealing treatment step of heating the titanium foil base on which the carbon film is formed in a vacuum or in an inert gas atmosphere;
In the annealing treatment step, after oxygen in the passivation film is diffused and absorbed in the titanium foil base material, carbon in the carbon film reacts with titanium in the titanium foil base material, and the titanium foil is A method for producing a titanium material for a fuel cell separator, characterized in that a conductive retaining coating is formed having a TiC layer in contact with one surface of a substrate. 8. The method for producing a titanium material for a fuel cell separator according to claim 7, wherein said carbon film is formed with a thickness of 5 nm or more and 30 nm or less in said carbon film forming step. The Fe content in the titanium foil base material before the carbon film forming step is 0.15% by mass or less, and the O content is 0.15% by mass or less. 9. The method for producing a titanium material for a fuel cell separator according to 8. The method for producing a titanium material for a fuel cell separator according to any one of claims 7 to 9, characterized in that the annealing temperature in said annealing step is 700°C or higher and 850°C or lower. 11. The method for producing a titanium material for a fuel cell separator according to any one of claims 7 to 10, wherein in said carbon film forming step, said carbon film is formed by an arc ion plating method. 12. The fuel cell according to any one of claims 7 to 11, further comprising a molding step of press-molding the titanium foil base on which the conductive retention film is formed after the annealing step. A method for producing a titanium material for a separator. A fuel cell separator included in a battery cell,
provided so as to sandwich an electrolyte membrane having electrode layers formed on both sides and a pair of gas diffusion layers provided on both sides of the electrolyte membrane from the outer surface side,
Having the titanium material for a fuel cell separator according to any one of claims 1 to 5,
A fuel cell separator, wherein the second surface of the titanium foil substrate is arranged to face the gas diffusion layer. A fuel cell comprising a plurality of stacked battery cells,
Each battery cell has the fuel cell separator according to claim 13 on its outer surface,
Adjacent fuel cell separators between the adjacent battery cells are in contact with each other at least partially, and cooling water flow paths are formed between the adjacent fuel cell separators,
A fuel cell characterized in that the conductive retaining film of the titanium material for fuel cell separators is present on the surfaces of the adjacent fuel cell separators facing each other.

Images