JP7306877B2 - METHOD FOR MANUFACTURING SEPARATOR MATERIAL FOR FUEL CELL - Google Patents

Description

TECHNICAL FIELD The present disclosure relates to a method for manufacturing a fuel cell separator material.

A fuel cell has a structure in which a solid polymer electrolyte membrane is sandwiched between an anode electrode and a cathode electrode as a single cell. Also, the fuel cell is configured as a stack in which a plurality of the unit cells are superimposed via a separator (also called a bipolar plate) having grooves for gas (hydrogen, oxygen, etc.) flow paths. Fuel cells can increase their output by increasing the number of cells per stack.

A separator for a fuel cell also plays a role of passing generated current to an adjacent cell through a surface through which cooling water (FCC) flows. Therefore, the separator material that constitutes the separator is required to have high conductivity and to maintain the high conductivity for a long period of time even in the atmosphere inside the cells of the fuel cell. Here, high conductivity means low contact resistance. Further, the contact resistance means that a voltage drop occurs due to an interfacial phenomenon between the electrode and the separator surface.

In order to satisfy such a demand, for example, Patent Document 1 discloses that a mixed layer in which titanium oxide and carbon black are mixed is formed on a substrate made of pure titanium or a titanium alloy, and the titanium oxide is crystalline. rutile, and 70% or more of the carbon detected when the bonding state of carbon in the mixed layer is analyzed by X-ray photoelectron spectroscopy is present as carbon black alone having a C—C bond. A fuel cell separator material characterized by the following is disclosed.

JP 2016-122642 A

In the technique described in Patent Document 1, a mixed layer containing titanium oxide and carbon black is formed on the surface of a titanium base material, and the carbon black is connected from the surface of the mixed layer to the interface between the mixed layer and the titanium base material. Thereby, conductivity is ensured. Since carbon black has excellent conductivity and titanium oxide has excellent corrosion resistance, it is described that the fuel cell separator material described in Patent Document 1 has high conductivity and conductive durability.

However, as described above, the separator material for fuel cells is required to have high conductivity. Separator materials are also required to have further improved conductivity.

Accordingly, an object of the present disclosure is to provide a method for producing a fuel cell separator material that can obtain a fuel cell separator material having excellent conductivity.

The present inventors have made intensive studies to solve the above problems, and have found that titanium oxide can be favorably formed in the subsequent oxidation treatment step by reducing the nitrogen concentration on the surface of the titanium base material to 2.0 atomic % or less. As a result, the inventors have found that the contact resistance can be reduced, leading to the present disclosure.

Therefore, an example of a mode of this embodiment is as follows.

[1] A method for producing a fuel cell separator material comprising a titanium base material and a surface layer containing a titanium oxide layer and carbon particles dispersed in the titanium oxide layer, the method comprising the steps of:
a nitrogen concentration reduction treatment step of reducing the nitrogen concentration on the surface of the titanium base material to 2.0 atomic % or less;
a coating step of coating carbon particles on the surface of the titanium base;
an oxidation treatment step of heat-treating the titanium base material in an oxidizing atmosphere to form the titanium oxide layer;
A method for producing a separator material for a fuel cell, comprising:
[2] The method for producing a fuel cell separator material according to [1], wherein in the nitrogen concentration reduction treatment step, the titanium base material is heat-treated in a vacuum atmosphere of 0.002 Pa or less for 30 seconds or longer.
[3] The method for producing a fuel cell separator material according to [2], wherein the temperature of the heat treatment is in the temperature range of 650 to 850°C.
[4] The method for producing a fuel cell separator material according to any one of [1] to [3], wherein the oxidizing atmosphere has a low oxygen partial pressure of 25 Pa or less.
[5] A separator material for a fuel cell comprising a titanium base material and a surface layer containing a titanium oxide layer and carbon particles dispersed in the titanium oxide layer,
A separator material for a fuel cell, wherein the nitrogen concentration on the surface of the titanium base material is 2.0 atomic % or less.

ADVANTAGE OF THE INVENTION By this indication, the manufacturing method of the separator material for fuel cells which can obtain the separator material for fuel cells excellent in electroconductivity can be provided.

1 is a schematic cross-sectional view for explaining the configuration of a fuel cell separator material obtained by the manufacturing method according to the present embodiment; FIG. 4 is a flow chart illustrating a method for manufacturing a fuel cell separator material according to the present embodiment. FIG. 4 is a schematic cross-sectional view showing a state in which carbon particles are applied to the surface of a titanium base material in a coating step; FIG. 3B is a schematic cross-sectional view showing a state after performing an oxidation treatment step following FIG. 3A; FIG. 2 is a schematic diagram for explaining the configuration of a device for measuring the contact resistance value of a test piece; 4 is a graph showing the results of measurement of contact resistance values of test pieces after an endurance test.

The present embodiment is a method for producing a fuel cell separator material comprising a surface layer containing a titanium oxide layer and carbon particles dispersed in the titanium oxide layer on a titanium base material, wherein the titanium base material is a nitrogen concentration reduction treatment step of reducing the nitrogen concentration on the surface of the titanium base material to 2.0 atomic % or less, a coating step of applying carbon particles to the surface of the titanium base material, and a heat treatment of the titanium base material in an oxidizing atmosphere. and an oxidation treatment step of forming the titanium oxide layer.

According to this embodiment, a fuel cell separator material having excellent conductivity can be produced. The reason why the conductivity is improved is only speculation and does not limit the present embodiment. It is considered that the titanium oxide layer can be satisfactorily formed in , and as a result, the contact resistance can be reduced. The reason why the titanium oxide layer can be formed satisfactorily is that by setting the nitrogen concentration on the surface of the titanium base material to 2.0 atomic % or less, the outward diffusion of titanium from the titanium base material to between the carbon particles is suppressed. It is presumed that it will be done well.

Hereinafter, the method for manufacturing the fuel cell separator material according to the present embodiment will be described in detail with reference to the drawings as appropriate.

<Separator material for fuel cells>
FIG. 1 is a schematic cross-sectional view illustrating the configuration of a fuel cell separator material obtained by the manufacturing method according to the present embodiment. As shown in FIG. 1, a fuel cell separator material 1 has a surface layer 3 formed on a titanium substrate 2 . The surface layer 3 includes a titanium oxide layer 4 and carbon particles 5 dispersed in the titanium oxide layer 4, as shown in FIG. When observing the cross section of the surface layer 3 , the carbon particles 5 are embedded in the titanium oxide layer 4 as the matrix of the surface layer 3 . The cross section may be a cross section along a plane parallel to the plane direction of the substrate, a cross section through a plane perpendicular to the plane direction, or a plane oblique to the plane direction. It may be a cross section by The carbon particles 5 are dispersed from the surface of the titanium oxide layer 4 to the interface between the titanium oxide layer 4 and the titanium substrate 2, and exist as a conductive path through which current flows.

A titanium substrate is a substrate composed of pure titanium or a titanium alloy. Examples of pure titanium include those defined in JIS H4600. Examples of titanium alloys include Ti--Al, Ti--Nb, Ti--Ta, Ti--6Al-4V, and Ti--Pd. However, in any case, it is not limited to these examples. A titanium base material made of pure titanium or a titanium alloy is light and has excellent corrosion resistance. In addition, even if there is an exposed portion on the surface of the titanium substrate that is not covered with the surface layer, the titanium or titanium alloy does not elute in the high-temperature acidic atmosphere (e.g., 80°C, pH 2) in the fuel cell. , there is no risk of deteriorating the solid polymer membrane.

A titanium base material is, for example, a cold-rolled material.

The thickness of the titanium substrate is, for example, 0.05-1 mm. When the thickness is within this range, it is easy to satisfy the demands for weight reduction and thinness of the separator, and the strength and handleability as a separator material are provided. Therefore, it becomes relatively easy to press the separator material into the shape of the separator. The shape of the titanium base material may be a long strip wound in a coil shape, or a sheet of paper cut to a predetermined size.

Carbon particles are particles composed of carbon, and include, for example, carbon black, graphite, B-doped diamond particles, N-doped diamond particles, and the like. One type of carbon particles may be used alone, or two or more types may be used in combination. Carbon black is carbon particles having a chain structure composed of amorphous carbon. Carbon black is classified into furnace black, acetylene black, thermal black, etc., depending on the production method, and any of them can be used. Graphite includes artificial graphite or natural graphite.

The average particle size of the carbon particles is preferably 20-200 nm. When the average particle size of the carbon particles is 20 nm or more, it becomes easier to suppress disappearance due to oxidation in the oxidation treatment step described later. Also, when the average particle diameter of the carbon particles is 200 nm or less, it becomes easier to retain them in the surface layer. The average particle size (primary particle size) is the average value of the diameters (equivalent circle diameters) of 100 randomly selected carbon particles in a transmission electron microscope image (TEM image).

The titanium oxide layer in the surface layer is made of, for example, titanium oxide represented by TiO _x (1<x≦2). From the viewpoint of electrical corrosion resistance, the titanium oxide layer preferably contains a crystalline rutile structure.

The thickness of the surface layer is preferably 10-500 nm. When the thickness of the surface layer is within this range, high conductivity and conductive corrosion resistance can be obtained.

A layer of carbon particles may be formed on the surface layer. This layer of carbon particles is, for example, the remaining carbon particles used to form the surface layer. Also, in one embodiment, such a layer of carbon particles is removed, such as by washing. Moreover, the surface layer may be formed only on one side of the titanium base material, or may be formed on both sides of the titanium base material.

<Method for producing fuel cell separator material>
FIG. 2 is a flow chart for explaining the method for manufacturing a fuel cell separator material according to this embodiment. As shown in FIG. 2, the fuel cell separator material manufacturing method according to the present embodiment includes a nitrogen concentration reduction treatment step S1, a coating step S2, and an oxidation treatment step S3.

(Nitrogen concentration reduction treatment step)
First, the manufacturing method according to the present embodiment includes a nitrogen concentration reduction treatment step for reducing the nitrogen concentration on the surface of the titanium base material to 2.0 atomic % or less.

The nitrogen concentration reduction treatment is not particularly limited as long as it can reduce the nitrogen concentration on the surface of the titanium base material to 2 atomic % or less. In one aspect of the present embodiment, in the nitrogen concentration reduction treatment step, the titanium base material is heat-treated in a vacuum atmosphere of 0.002 Pa or less for 30 seconds or more. By annealing the titanium base material under such conditions, the nitrogen existing on the surface of the titanium base material can be diffused and the nitrogen concentration on the surface of the titanium base material can be sufficiently reduced. Also, by annealing in a vacuum atmosphere of 0.002 Pa or less, nitrogen can be prevented from entering the titanium base material. Further, in one aspect of the present embodiment, the titanium base material is heat-treated in an argon gas-containing atmosphere in the nitrogen concentration reduction treatment step. By annealing in an atmosphere containing argon gas, the nitrogen concentration in the titanium base material can be reduced while suppressing nitrogen contamination.

The temperature of the heat treatment is preferably in the temperature range of 650-850.degree. Nitrogen can be sufficiently diffused by setting the temperature of the heat treatment to 650° C. or higher. Further, by setting the heat treatment temperature to 850° C. or lower, it is possible to suppress the change of titanium from α phase to β phase.

The nitrogen concentration on the surface of the titanium base material after the nitrogen concentration reduction treatment step is 2.0 atomic % or less, preferably 1.8 atomic % or less, more preferably 1.6 atomic % or less, and further Preferably, it is 1.4 atomic % or less. In the present embodiment, the nitrogen concentration on the surface of the titanium base refers to the nitrogen concentration at a depth of 10 nm from the outermost surface of the titanium base. The nitrogen concentration can be measured using an X-ray Photoelectron Spectroscopy (XPS). Composition analysis in the depth direction can be performed by using XPS. In one aspect of the present embodiment, the average nitrogen concentration from the outermost surface of the titanium substrate to a depth of 50 nm is also 2.0 atomic % or less.

In general, organic matter present in the atmosphere may be adsorbed on the surface of the titanium base material, and the presence of such organic matter is preferably removed by washing or the like.

(Coating process)
Next, the manufacturing method according to the present embodiment includes a coating step of coating the surface of the titanium base material with carbon particles. FIG. 3A is a schematic diagram showing a state in which the carbon particles 5 are applied to the surface of the titanium base material 2 in the application step S2.

The carbon particles can be applied onto the titanium substrate in the form of an aqueous or oily dispersion (also referred to as dispersion paint) in which the carbon particles are dispersed. The carbon particles can also be applied directly onto the titanium substrate.

A dispersion coating containing carbon particles may contain a binder resin and/or a surfactant. However, binder resins and surfactants tend to lower the electrical conductivity, so the content of these is preferably as small as possible. The dispersion paint can also contain other additives, if desired.

As the binder resin, it is preferable to use a resin that decomposes without residue when heated in the oxidation treatment step. Examples of such binder resins include acrylic resins, polyethylene resins, polypropylene resins, polystyrene resins, polyvinyl alcohol resins, and the like. Among these, acrylic resins are preferred from the viewpoint that the lower the temperature at which they decompose, the less they affect the formation of the surface layer. Binder resin may be used individually by 1 type, and may use 2 or more types together.

The blending ratio of the carbon particles and the binder resin in the dispersion paint is preferably 0.3 to 2.5 in terms of mass ratio of the solid content (binder resin solid content/carbon particle solid content). The smaller the mass ratio, the greater the amount of carbon particles, resulting in improved electrical conductivity. Therefore, from the viewpoint of conductivity, this mass ratio is preferably 2.5 or less, more preferably 2.3 or less. On the other hand, the larger the mass ratio, the larger the amount of the binder resin. Therefore, when this mass ratio is large, the adhesion between the titanium base material 2 and the coating film is increased. Therefore, from the viewpoint of adhesion, this mass ratio is preferably 0.3 or more, more preferably 0.4 or more.

As the aqueous medium, for example, water or ethanol can be used. For example, toluene, cyclohexanone, or the like can be used as the oily medium.

The average particle size of the carbon particles is preferably 20-200 nm. Since carbon particles tend to form agglomerates in paint, it is preferable to use a paint devised to prevent the formation of agglomerates. For example, as the carbon particles, it is preferable to use carbon black in which a functional group such as a carboxyl group is chemically bonded to the surface to enhance the repulsion between the particles, thereby enhancing the dispersibility.

The amount of carbon particles to be applied to the surface of the titanium base material is not particularly limited, and can be appropriately selected in consideration of conductivity and the like. From the viewpoint of conductivity, the coating amount of the carbon particles is preferably 1.0 μg/cm ² or more, more preferably 2.0 μg/cm ² or more. The amount of carbon particles to be applied is preferably 50 μg/cm ² or less. Even if the coating amount of the carbon particles is larger than this, the effect of improving the conductivity tends to be saturated.

Examples of methods for applying the dispersion containing carbon particles to the titanium substrate include brush coating, bar coaters, roll coaters, gravure coaters, die coaters, dip coaters, and spray coaters. is not limited to Further, as a method of applying in the form of powder, for example, a method of using a toner produced using carbon particles and electrostatically coating the toner onto a titanium base material can be mentioned.

(Oxidation treatment step)
Next, the manufacturing method according to the present embodiment includes an oxidation treatment step of heat-treating the titanium base material in an oxidizing atmosphere to form a titanium oxide layer. FIG. 3B is a schematic cross-sectional view showing a state in which the titanium oxide layer 4 is formed on the surface of the titanium base material 2 by the oxidation treatment step S3. In the oxidation treatment step S3, when the titanium base material 2 coated with the carbon particles 5 is heat treated in an oxidizing atmosphere, the titanium in the titanium base material 2 diffuses outward between the carbon particles 5, and the outward diffusion Part or all of the deposited titanium is oxidized to form a titanium oxide layer 4 . As a result, the surface layer 3 in which the carbon particles 5 are dispersed in the titanium oxide layer 4 is formed.

The oxidizing atmosphere is not particularly limited as long as it is an atmosphere in which titanium is oxidized by heat treatment to form titanium oxide, but the oxidizing atmosphere preferably has a low oxygen partial pressure of 25 Pa or less. If the oxygen partial pressure in the oxidation treatment step S3 exceeds 25 Pa, the carbon particles may burn into carbon dioxide and disappear. In addition, the carbon particles are oxidatively decomposed, and titanium is excessively oxidized at the exposed surface of the titanium base material, which may cause the titanium oxide layer 4 to become too thick. Therefore, the oxygen partial pressure is preferably 25 Pa or less, more preferably 20 Pa or less, even more preferably 15 Pa or less, and particularly preferably 10 Pa or less. The oxygen partial pressure can be appropriately adjusted by reducing the pressure or by using an inert gas such as Ar gas or nitrogen gas. From the viewpoint of promoting oxidation, the oxygen partial pressure is preferably 0.05 Pa or higher, more preferably 0.1 Pa or higher, and even more preferably 0.5 Pa or higher. The heat treatment temperature is, for example, in the temperature range of 300 to 800.degree. C., preferably 500 to 750.degree. When the oxygen partial pressure and the heat treatment temperature are within the ranges described above, part or all of the titanium atoms outwardly diffused from the titanium base material 2 react with a small amount of oxygen in the atmosphere to form titanium oxide. The surface layer 3 mixed with carbon particles can be easily formed.

The heat treatment time can be appropriately selected in consideration of conditions such as heat treatment temperature and oxygen partial pressure. The heat treatment time is, for example, 1 to 60 minutes when the heat treatment temperature is 500.degree. C., and 10 to 120 seconds when the temperature is 700.degree.

The method for producing the fuel cell separator material according to the present embodiment is as described above, but the method for producing the fuel cell separator material according to the present embodiment may optionally include steps other than the steps described above. can. For example, a pickling treatment step may be included before the nitrogen concentration reduction treatment step S1. Further, for example, a degreasing step for removing rolling oil may be included after the rolling step. A drying step for drying the coated surface may be included between the coating step S2 and the oxidation treatment step S3. Further, after the oxidation treatment step S3, a straightening step (leveling step) may be included to straighten the longitudinal warp of the titanium base material caused by the heat treatment and flatten it. In addition, correction can be performed by using a tension leveler, a roller leveler, or a stretcher, for example. Further, a washing/drying step of washing and drying the fuel cell separator material that has undergone the oxidation treatment step S3 or the straightening step may be included. The washing may remove excess carbon particles present on the surface layer. The method may also include a cutting step of cutting the fuel cell separator material, which has undergone the above steps, into a predetermined size. All of these steps are optional steps and can be performed as necessary.

The method for producing the fuel cell separator material according to the present embodiment is as described above. The fuel cell separator material according to the present embodiment includes a titanium oxide layer and a A fuel cell separator material comprising a surface layer containing dispersed carbon particles, wherein the nitrogen concentration on the surface of the titanium base material is 2.0 atomic % or less. .

<Manufacturing method of fuel cell separator>
In order to produce a fuel cell separator using the fuel cell separator material according to the present embodiment, a gas flow path through which gas flows and a gas that is introduced into the gas flow path are provided in the fuel cell separator material. It is preferable to carry out a press molding step for forming the introduction port.

Press-molding can be performed, for example, using a press-molding apparatus equipped with a molding die having a desired shape (for example, a molding die for forming a gas flow path and a gas inlet). In addition, you may use a lubricant at the time of shaping|molding as needed. When press molding is performed using a lubricant, it is preferable to perform a step for removing the lubricant after the press molding step.

The present embodiment will be described below using examples.

[Example 1]
(Base material)
A cold-rolled material (size: 20×65 mm) of pure titanium (type 1 specified in JIS H 4600) having a thickness of 0.1 mm was used as the base material.

(Nitrogen concentration reduction treatment step)
The above titanium substrates were annealed in a furnace under the conditions shown in Table 1 in an argon gas-containing atmosphere to produce substrates E1 to E3 and C1 to C7. In this example and comparative example, the argon gas concentration in the furnace was adjusted by adjusting the discharge amount of argon gas. Table 1 shows the argon gas discharge amount in each example or comparative example as a relative ratio, with the argon gas discharge amount in Example 1 being 100. The nitrogen concentration at a depth of 10 nm from the outermost surface of the substrates E1 to E3 and C1 to C7 thus produced was measured by XPS. Table 1 shows the results.

(Application process: application of carbon particle dispersion paint)
As the carbon particles, a commercially available carbon black-containing paint (Aqua Black-162, manufactured by Tokai Carbon Co., Ltd.) was used. After diluting this paint with distilled water and ethanol, an acrylic resin was added to prepare a carbon black dispersion paint. Then, this carbon black-dispersed paint was applied to both sides of the substrate using a bar coater. The coating amount was adjusted so that the carbon black ranged from 20 to 30 μg/cm ² .

(Oxidation treatment step)
The oxidation treatment was performed using a heat treatment furnace equipped with a sample chamber and a heating chamber. First, the base material coated with the carbon black dispersion paint was placed in the sample chamber of the heat treatment furnace. Next, the inside of the furnace was evacuated to 0.01 Pa or less with a vacuum pump. Next, the temperature of the heating chamber was raised to 650°C. Oxygen was then introduced into the furnace. Next, the titanium substrate was transported from the sample chamber to the heating chamber and heated at 650°C. The titanium substrate was then returned to the sample chamber and cooled.

(Washing process)
After the titanium base material was cooled to 100° C. or less, the pressure inside the furnace was returned to atmospheric pressure, and the titanium base material was taken out. Thereafter, excess carbon black present on the surface of the titanium substrate was wiped off with gauze impregnated with ethanol. The substrate was then ultrasonically cleaned in ethanol.

Through the above steps, test pieces E1 to E3 and C1 to C7 were produced.

[evaluation]
Each test piece was subjected to the following durability test, and the contact resistance value of the test piece after the durability test was measured.

(An endurance test)
Each test piece was immersed in a fuel cell coolant at 80° C. for 100 hours. A mixture of water and ethylene glycol at a ratio of 1:1 was used as the coolant.

(Measurement of contact resistance value)
The contact resistance value was measured using the contact resistance measuring device 10 shown in FIG. Specifically, both sides of the test piece 11 are sandwiched between carbon cloths 12 (CC6 Plain manufactured by Fuel Cell Earth, thickness 26 mils (approximately 660 μm)), and the outside thereof is further covered with two copper electrodes 13 having a contact area of 1 cm ² . It was sandwiched and pressurized with a load of 98 N (10 kgf). Then, a current of 7.4 mA was applied using the direct current power supply 14, and the voltage applied between the carbon cloths 12 was measured by the voltmeter 15 to obtain the contact resistance value. A test piece having a contact resistance value of 7 mΩ·cm ² or less after the endurance test was judged to be a non-defective product.

Table 1 and FIG. 5 show the results of measuring the contact resistance values of the test pieces after the endurance test.

The test pieces E1 to E3 had very low contact resistance values of 5 mΩ·cm ² or less, showing good results.

REFERENCE SIGNS LIST 1 fuel cell separator material 2 titanium substrate 3 surface layer 4 titanium oxide layer 5 carbon particles S1 application step S2 oxidation treatment step S3 reduction treatment step

Claims (2)

A method for producing a fuel cell separator material comprising a titanium base material and a surface layer containing a titanium oxide layer and carbon particles dispersed in the titanium oxide layer, the method comprising the steps of:
a nitrogen concentration reduction treatment step of reducing the nitrogen concentration on the surface of the titanium base material to 2.0 atomic % or less;
a coating step of coating carbon particles on the surface of the titanium base;
an oxidation treatment step of heat-treating the titanium base material in an oxidizing atmosphere to form the titanium oxide layer;
A method for producing a separator material for a fuel cell, comprising: A fuel cell separator material comprising a titanium base material and a surface layer containing a titanium oxide layer and carbon particles dispersed in the titanium oxide layer,
A separator material for a fuel cell, wherein the nitrogen concentration on the surface of the titanium base material is 2.0 atomic % or less.

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