JP6878239B2 - Manufacturing method of separator for fuel cell - Google Patents

Description

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

In a fuel cell, a solid polymer electrolyte membrane sandwiched between an anode electrode and a cathode electrode is used as a single cell, and the single cell is formed through a separator in which a flow path for gas (hydrogen gas, oxygen gas, etc.) is formed. It is configured as a stack of multiple layers. Since the separator for a fuel cell plays a role of passing the current generated in the stack to the adjacent cells, high conductivity and conductive durability are required.

As a method for producing a separator for a fuel cell, for example, in Patent Document 1, carbon black is applied to the surface of a titanium base material having a carbon concentration of 10 atomic% or less at a depth of 10 nm from the surface. A production method for heat-treating the resulting base material under a low oxygen partial pressure having an oxygen partial pressure of 25 Pa or less is disclosed.

In this method, the titanium atoms of the base material diffuse outward into the carbon black and react with a small amount of oxygen gas in a low oxygen atmosphere to form titanium oxide, forming a mixed layer of titanium oxide and carbon black. .. By forming such a mixed layer, high conductivity and conductive durability can be imparted to the separator.

Japanese Unexamined Patent Publication No. 2016-122642

However, the separator produced by the method described in Patent Document 1 is crystallinely stable on the surface of the mixed layer due to the reaction between the titanium atoms diffused outward and the trace amount of oxygen atoms on the surface of the mixed layer. A certain titanium oxide (TiO ₂ ) is formed. Although such a separator has high conductivity and conductive durability, it has low adhesive strength with an adhesive used for bonding to a membrane electrode assembly of a fuel cell, for example.

Further, as described above, _{titanium oxide having a composition formula of TiO 2} is formed on the surface of the mixed layer, but since it is not in contact with oxygen gas inside the mixed layer, the titanium base material is formed from the surface of the mixed layer. The oxygen concentration of titanium oxide decreases as it approaches the interface of. Due to such a decrease in oxygen concentration, a difference in strength occurs in the mixed layer, which may reduce the strength of the separator.

The present invention has been made in view of the above points, and provides a method for manufacturing a separator for a fuel cell capable of improving the strength of the mixed layer and improving the adhesive force between the surface of the mixed layer and the adhesive. To do.

As a result of diligent studies by the inventors in order to solve the above problems, the oxygen atom of titanium oxide on the surface of the mixed layer is diffused inside the mixed layer to (1) homogenize the mixed layer. It was thought that the strength could be stabilized and (2) the oxygen deficiency of titanium oxide on the surface of the mixed layer could activate the surface and improve the adhesive force with the adhesive.

The present invention has been made based on such an idea, and the method for producing a separator for a fuel cell according to the present invention has a carbon concentration of 10 atomic% or less located at a depth of 10 nm from the surface. , A step of preparing a titanium base material made of titanium or a titanium alloy, a step of applying carbon black to the surface of the titanium base material, and a step of applying oxygen partial pressure to the titanium base material to which the carbon black is applied. By performing the heat treatment in a low oxygen atmosphere containing oxygen gas at 25 Pa or less, the titanium atoms of the titanium base material are outwardly diffused into the carbon black, and the outwardly diffused titanium atoms and the oxygen gas are combined with each other. To produce titanium oxide and form a mixed layer made of the titanium oxide and the carbon black, and the titanium base material on which the mixed layer is formed is heat-treated in a vacuum atmosphere. _{It is characterized by including a step of producing titanium oxide of TiO x} (where x is 1.3 to 1.7 in the formula) on the surface of the mixed layer.

In the separator for a fuel cell according to the present invention, the surface of a titanium base material made of titanium or a titanium alloy is coated with a mixed layer made of titanium oxide and carbon black, and the surface of the mixed layer is coated with TiO _x (formula). Among them, x is 1.3 to 1.7), which is characterized by being coated with titanium oxide.

In the present invention, a titanium base material on which a mixed layer composed of titanium oxide produced by outward diffusion and carbon black is formed is heat-treated in a vacuum atmosphere. As a result, the oxygen atoms bonded to the titanium atoms can be diffused inside the mixed layer while suppressing the oxidation of the titanium atoms on the surface of the mixed layer. As a result, the surface of the mixed layer becomes an active state having unpaired electrons due to oxygen deficiency of titanium oxide, so that the adhesive force between the adhesive and the surface of the mixed layer is improved.

Further, according to the present invention, oxygen atoms are diffused inside the mixed layer from the surface of the mixed layer to the interface with the titanium base material so that the oxygen concentration (oxygen atom composition ratio) bonded to titanium oxide becomes substantially uniform. Therefore, it is possible to suppress a decrease in the strength of the mixed layer due to the difference in oxygen concentration. As a result, it is possible to obtain a separator having an improved adhesive force with an adhesive while increasing the strength of the mixed layer.

(A) is a schematic cross-sectional view of a separator after the step of forming a mixed layer made of titanium oxide and carbon black and before heat treatment is performed in a vacuum atmosphere, and (b) is a schematic cross-sectional view. It is a figure explaining the electronic state of the mixed layer formed in the separator of). (A) is a schematic cross-sectional view of a separator after heat treatment in a vacuum atmosphere, and (b) is a diagram for explaining an electronic state of a mixed layer formed on the separator of (a). It is a figure which shows the spectrum by the TEM (Transmission Electron Microscopy) -EELS (Electron Energy Loss Spectroscopy) analysis which concerns on an Example and a comparative example.

Hereinafter, the separator 1 according to the embodiment of the present invention will be described with reference to FIGS. 1 and 2, and the manufacturing method thereof will be described.

FIG. 1A shows the separator 1 after the step of forming the mixed layer 3A composed of titanium oxide 32 and carbon black 31 by the heat treatment in a low oxygen atmosphere, and before the heat treatment in a vacuum atmosphere. It is a schematic cross-sectional view of. FIG. 1B is a diagram illustrating a state of electrons in the mixed layer 3A formed on the separator 1 of FIG. 1A.

FIG. 2A is a schematic cross-sectional view of the separator 1 after heat treatment in a vacuum atmosphere. FIG. 2B is a diagram illustrating an electronic state of the mixed layer 3B of the separator 1 of FIG. 2A.

As shown in FIG. 2A, the separator 1 according to the present embodiment is used for a cell of a fuel cell, and a mixed layer 3B is formed on the surface 21 of the titanium base material 2. The mixed layer 3B is composed of titanium oxide 32 and carbon black 31. The titanium oxide 33 on the surface 35 of the mixed layer 3B corresponding to the surface of the separator 1 _{is TIO x} (x is 1.3 to 1.7 in the formula), and is titanium oxide having an oxygen deficiency.

Here, when the value of x in the formula is less than 1.3, it cannot be said that the strength of the mixed layer 3B is sufficient because the number of oxygen atoms bonded to Ti of the mixed layer 3B is too small. On the other hand, when the value of x exceeds 1.7, the activity of the surface 35 of the mixed layer 3B is not sufficient and the adhesiveness between the surface 35 of the mixed layer 3B and the adhesive is not sufficient, as will be described later. It will drop. The manufacturing method of the separator 1 will be described below.

1. 1. Regarding the Step of Preparing the Titanium Base Material 2 (First Step) In the present embodiment, first, a titanium group made of titanium or a titanium alloy having a carbon concentration of 10 atomic% or less located at a depth of 10 nm from the surface 21 Prepare material 2.

The carbon concentration at a depth of 10 nm from the surface 21 of the titanium base material 2 is subjected to composition analysis in the depth direction using, for example, an X-ray Photoelectron Spectroscopy (XPS). It can be measured by.

The titanium base material 2 has a carbon concentration located at a depth of 10 nm from the surface 21 because a part of carbon atoms contained in rolling oil during rolling or lubricating oil during molding may diffuse to the surface layer thereof. However, it may exceed 10 atomic%.

When the carbon concentration exceeds 10 atomic% at a depth of 10 nm from the surface 21 of the titanium base material 2, a region contaminated with organic substances or a region in which titanium carbide is formed is formed on the surface 21 of the titanium base material 2. Very likely to exist. When such a titanium base material 2 is used, it becomes difficult for the carbon black 31 to bond to the surface of the titanium base material 2 in the heat treatment in a low oxygen atmosphere, which will be described later, and as a result, the outward diffusion of titanium atoms is hindered. ..

Therefore, in the present embodiment, when the carbon concentration exceeds 10 atomic% at a depth of 10 nm from the surface 21 of the titanium base material 2, the surface has a carbon concentration of 10 atomic% or less at that position. Perform processing. As such a surface treatment, for example, the titanium base material 2 is pickled in an acidic aqueous solution containing hydrofluoric acid. In addition to this, for example, carbon may be diffused into the titanium base material 2 by heat treatment at a temperature of 650 ° C. or higher in a vacuum atmosphere, and a layer having a high carbon concentration is physically formed by shot blasting or polishing. May be removed.

Examples of the titanium of the titanium base material 2 to be prepared include 1 to 4 types specified in JIS H 4600. Examples of the titanium alloy include Ti-Al, Ti-Nb, Ti-Ta, Ti-6Al-4V, and Ti-Pd. However, in any case, the present invention is not limited to the above examples. When a base material made of titanium or a titanium alloy is used, it can be made light and have excellent corrosion resistance.

As the material of the titanium base material 2, for example, a plate material cold-rolled to a thickness of 0.05 to 1 mm is preferable. When the thickness is within this range, the requirements for weight reduction and thinning of the separator 1 are satisfied, the strength and handleability of the separator 1 are provided, and it is relatively easy to press work into the shape of the separator 1.

2. About the step of applying carbon black (second step) In this step, carbon black 31 is applied to the surface 21 of the titanium base material 2 prepared in the first step. Specifically, an aqueous or oily liquid (dispersion liquid) in which the powder of carbon black 31 is dispersed is applied, or the powder of carbon black 31 is directly applied. As the aqueous liquid, for example, water can be used as a solvent. As the oily liquid, for example, ethanol, toluene, cyclohexanone and the like can be used.

The particle size of the carbon black 31 powder is preferably 20 to 200 nm. When the carbon black 31 powder easily forms an agglomerate, for example, it is preferable to use the carbon black 31 powder in which a functional group such as a carboxyl group is chemically bonded to the surface. By preparing the dispersion liquid with such carbon black 31 powder, the repulsion between the carbon black 31 powders can be strengthened and the dispersibility can be enhanced, so that the formation of aggregates can be suppressed.

The lower limit of the amount of the carbon black 31 powder applied to the surface 21 of the titanium base material 2 ^{is preferably 1 μg / cm 2} or more. By setting such a lower limit value, high conductivity and conductive durability can be obtained. On the other hand, the upper limit of the coating amount is preferably about ^{50 μg / cm 2.} By setting such an upper limit value, it is possible to suppress an increase in cost.

As a method of applying the dispersion liquid in which the powder of carbon black 31 is dispersed to the titanium base material 2, for example, brush coating, bar coater, roll coater, gravure coater, die coater, dip coater, spray coater or the like is used. However, it is not limited to these. Further, when it is applied in the form of powder, it can be applied by electrostatically coating the titanium base material 2 with a toner prepared by using carbon black 31.

3. 3. About the step of forming the mixed layer 3A (third step) In this step, as shown in FIG. 1A, the oxygen partial pressure is applied to the titanium base material 2 coated with carbon black 31 in the second step. The heat treatment is performed at 25 Pa or less in a low oxygen atmosphere containing oxygen gas. As a result, the titanium atoms of the titanium base material 2 are outwardly diffused into the carbon black 31, and the outwardly diffused titanium atoms are reacted with oxygen gas to generate titanium oxide 32, so that the titanium oxide 32 and the carbon black are produced. A mixed layer 3A composed of 31 is formed.

Here, when the oxygen partial pressure exceeds 25 Pa, the carbon of the carbon black 31 may react with the oxygen gas to become carbon dioxide (combustion). Therefore, it is difficult to form a mixed layer 3A composed of titanium oxide 32 produced by reacting the outwardly diffused titanium atoms with oxygen gas and carbon black 31.

The oxygen partial pressure is preferably in the range of ^{1.3 × 10 -3 to 21 Pa, for example.} Further, the temperature of the heat treatment is preferably in the temperature range of, for example, 300 to 800 ° C. When the oxygen partial pressure and the heat treatment temperature are in the above ranges, a part or all of the titanium atoms diffused outward from the titanium base material 2 react with a trace amount of oxygen in the atmosphere to become titanium oxide, and titanium oxide is formed. The mixed layer 3A in which 32 and carbon black 31 are mixed can be reliably formed.

As shown in FIG. 1 (b), the surface 35 of the mixed layer 3A corresponding to the surface of the separator 1 obtained in this step is crystalline having a rutile-type crystal structure represented by the _{composition formula TiO 2.} There is stable titanium oxide in. From the surface 35 of the mixed layer 3A to the interface 34 with the titanium base material 2 (that is, from the surface 35 of the mixed layer 3A toward the thickness of the mixed layer 3A), the oxygen concentration bonded to the titanium atom decreases. To do. Then, at the interface 34, TiO _{x having} a low oxygen concentration (x is about 1 in the formula) may be present.

_{When titanium oxide (TiO 2} ) is present on the surface 35 of such a separator 1 (mixed layer 3A), titanium _{oxide (TiO 2} ) has a stable crystal structure, and therefore titanium atoms are elements that constitute an adhesive. The bond energy between the titanium atom and the oxygen atom is larger than the energy that binds to. Therefore, the adhesive force between the surface 35 of the mixed layer 3A and the adhesive is low.

Further, on the surface 35 of the mixed layer 3A, _{titanium oxide represented by the composition formula TiO 2} is formed, but since the inside of the mixed layer 3A is not in contact with oxygen gas, as described above, the mixed layer 3A The oxygen concentration in titanium oxide decreases as the surface 35 advances to the interface 34 with the titanium substrate 2. As a result, the strength of the mixed layer 3A may decrease due to the difference in the oxygen concentration of titanium oxide.

Therefore, in the present embodiment, after heat treatment is performed in a low oxygen atmosphere in order to diffuse the oxygen atoms of the surface 35 of the mixed layer 3A into the inside of the mixed layer 3A while suppressing the oxidation of the surface 35 of the mixed layer 3A. Further, the heat treatment is further performed in a vacuum atmosphere. The details of each step of the manufacturing method of this embodiment will be described below.

4. About the step (fourth step) of producing TiO _x (x: 1.3 to 1.7) In this step, as shown in FIG. 2 (a), with respect to the titanium base material 2 obtained in the third step. Then, by performing the heat treatment in a vacuum atmosphere, _{titanium oxide 33 of TiO x} (x is 1.3 to 1.7 in the formula) is produced on the surface 35 of the mixed layer 3B. Thereby, the separator 1 for the fuel cell according to the present embodiment can be obtained. That is, in the separator 1 for a fuel cell, the surface of a titanium base material 2 made of titanium or a titanium alloy is coated (formed) with a mixed layer 3B made of titanium oxide 32 and carbon black, and the surface of the mixed layer 3B is coated (formed). , _{Titanium oxide 33 of TiO x} (where x is 1.3 to 1.7 in the formula) is coated (formed).

In the present embodiment, since the heat treatment is performed in a vacuum atmosphere, the oxygen atoms bonded to the titanium atoms can be diffused inside while suppressing the oxidation of the titanium atoms on the surface 35 side of the mixed layer 3B. As a result, _{titanium oxide 33 which becomes TiO x} (x is 1.3 to 1.7 in the formula) can be produced on the surface 35 of the mixed layer 3B, and the titanium oxide contained in the mixed layer 3B can be produced. The difference in oxygen concentration of 32 can be suppressed.

Here, the vacuum atmosphere is ^{a pressure atmosphere of at least 1.3 × 10 -4} Pa and is in a state of an oxygen-free atmosphere. The conditions for the heat treatment performed in a vacuum atmosphere are a heating temperature of 300 to 840 ° C. and a treatment time of 10 to 60 seconds. By performing the treatment in a vacuum atmosphere within the range of such heating temperature and treatment time, the composition ratio (value of x) of the oxygen atom of _{titanium oxide TiO x formed on the surface 35 of the mixed layer 3B is set to 1.} It can be in the range of .3 to 1.7.

As shown in FIG. 2B, the surface 35 of the mixed layer 3B of the separator 1 obtained in this step has titanium unpaired electrons and is in an electronically unstable state, so that it is stable together with the adhesive. The adhesive strength between the surface 35 of the mixed layer 3B and the adhesive is improved.

Here, examples of the adhesive include a thermoplastic adhesive such as polypropylene (PP), polyethylene (PE), and polyethylene terephthalate (PET), a thermosetting adhesive such as silicone and epoxy, and an acrylic two-component type. Adhesives and rubber elastomers such as epilene propylene diene rubber (EPDM), nitrile rubber (NBR), and fluororubber can be mentioned.

According to the present embodiment, as described above, the titanium base material 2 (see FIG. 1A) on which the mixed layer 3A composed of the titanium oxide 32 produced by outward diffusion and the carbon black 31 is formed. On the other hand, the heat treatment was performed in a vacuum atmosphere. As a result, the oxygen atoms bonded to the titanium atoms are internally diffused while suppressing the oxidation of the titanium atoms on the surface 35 of the mixed layer 3A. Therefore, the concentration difference of the oxygen atom bonded to the titanium atom inside the mixed layer 3B becomes small, and as a result, the strength of the mixed layer 3B of the separator 1 is improved.

Further, the titanium oxide on the surface 35 of the mixed layer 3B is in an active state having titanium unpaired electrons due to oxygen deficiency (see FIG. 2B), so that the surface of the mixed layer 3B, which is the surface of the separator 1, is formed. The adhesive force between the 35 and the adhesive applied to the surface 35 is improved.

Hereinafter, the present invention will be described with reference to Examples.

<Example>
A test body according to Example 1 was produced as a test body corresponding to the separator of the present invention according to the first to fourth steps described above.

[First step]
First, a titanium base material having a carbon concentration of 10 atomic% or less located at a depth of 10 nm from the surface was prepared. The prepared titanium base material is a cold-rolled material of pure titanium (one type specified in JIS H 4600), the size of the titanium base material is 50 × 150 mm, and the thickness is 0.1 mm. It was. As a result of measuring the carbon concentration at a depth of 10 nm from the surface of the base material by XPS analysis, it was 5 atomic% or less.

[Second step]
Next, a treatment of applying carbon black to the surface of the titanium base material was performed. A commercially available paint (Aqua Black-162 manufactured by Tokai Carbon Co., Ltd.) was used as the paint in which carbon black was dispersed. The paint was appropriately diluted with distilled water and ethanol, and applied onto the substrate by brush coating. The coating amount was 30 μg / cm ² .

[Third step]
Next, a 20 × 50 mm test piece was cut out from the titanium base material coated with carbon black obtained in the second step. By heat-treating this test piece in a low oxygen atmosphere containing oxygen gas at an oxygen partial pressure of 10 Pa (temperature 700 ° C., treatment time 30 seconds), the titanium atoms of the titanium base material were put into carbon black. Along with outward diffusion, titanium oxide was produced by reacting the outwardly diffused titanium atoms with oxygen gas to form a mixed layer composed of titanium oxide and carbon black.

[Fourth step]
Next, the test piece obtained in the third step was heat-treated (temperature 750 ° C., treatment time 15 seconds) in a vacuum atmosphere. This heat treatment was carried out using the vacuum heat treatment furnace used in the third step. The obtained test body was used as an example.

<Comparison example>
A test body prepared in the same manner as in the examples except that the above-mentioned fourth step was not performed was used as a comparative example.

<TEM-EELS analysis>
The valence of titanium oxide was measured by analyzing the compositional state of the surface of the mixed layer of Examples and Comparative Examples by TEM-EELS.

<Results and discussion>
FIG. 3 is a diagram showing spectra obtained by TEM-EELS analysis according to Examples and Comparative Examples. Spectral lines A and B are spectrum lines of Examples (after the vacuum heat treatment of the fourth step) and Comparative Examples (before the vacuum heat treatment of the fourth step), respectively. ▼ in the figure indicates the peaks of the respective spectral lines A and B.

In the TEM-EELS measurement, the lower the measured energy loss, the more oxygen deficiency is present in titanium oxide. As can be seen from FIG. 3, the energy loss on the surface of the mixed layer decreased from 463.0 eV to 462.2 eV before and after the heat treatment in a vacuum atmosphere.

From this, before the heat treatment under the vacuum atmosphere, TiO ₂ was formed on the surface of the mixed layer, and after the heat treatment under the vacuum atmosphere, the titanium oxide having oxygen deficiency was formed on the surface of the mixed layer. It is thought that it has been formed. Then, from the measured energy loss, it was found that the titanium oxide on the surface of the mixed layer was _{about TiO 1.5.}

When the heating temperature in a vacuum atmosphere is in the range of 300 to 840 ° C. and the processing time is in the range of 10 to 60 seconds, _{the value of x of TiO x} is in the range of 1.3 to 1.7. I was able to confirm.

Further, for the examples, a plurality of test bodies before vacuum heating before performing the fourth step and a plurality of test bodies after vacuum heating after performing the fourth step were prepared. Then, a modified polypropylene adhesive (adhesive tape) having a thickness of 30 μm is placed on the surface of these test specimens at 160 ° C. for 3 seconds under the condition of a pushing amount of 5 μm, and then a force to peel it off (initial adhesion). Force) was measured. The results are shown in the left column of Table 1. If the adhesive strength is 0.6 N / mm or more, the adhesive state is good.

Further, for the examples, a plurality of test bodies before vacuum heating before performing the fourth step and a plurality of test bodies after vacuum heating after performing the fourth step were prepared, and 95 for these test pieces. A durability test was conducted in which the product was immersed in warm water at ° C. for 100 hours. After that, an adhesive made of modified polypropylene (adhesive tape having) is attached to the surface of these specimens under the same conditions as described above, and then the force to peel this off from the surface of the specimens (after the durability test). Adhesive strength) was measured. The results are shown in the right column of Table 1.

From this result, as in the present invention, by performing the fourth step, the adhesive strength of the test piece after vacuum heating is improved, and further, the adhesive strength is 0.6 N / mm or more (specifically) even after the durability test. Was able to be secured at 0.6 to 0.8 N / mm). On the other hand, the test piece before vacuum heating, which has not undergone the fourth step, has an adhesive force of 0.6 N / mm or less (specifically, 0.2 to 0.6 N / mm) after the durability test. Adhesive strength was significantly reduced.

Although one embodiment of the present invention has been described in detail above, the present invention is not limited to the above-described embodiment, and various types are described within the scope of the claims as long as the spirit of the present invention is not deviated. It is possible to make design changes.

1: Separator 2: Titanium base material, 3A, 3B: Mixed layer, 31: Carbon black, 32, 33: Titanium oxide

Claims (2)

A method for manufacturing separators for fuel cells.
A step of preparing a titanium base material made of titanium or a titanium alloy having a carbon concentration of 10 atomic% or less located at a depth of 10 nm from the surface, and
The process of applying carbon black to the surface of the titanium base material and
By heat-treating the titanium base material coated with the carbon black in a low oxygen atmosphere containing oxygen gas at an oxygen partial pressure of 25 Pa or less, the titanium atoms of the titanium base material are contained in the carbon black. A step of forming a mixed layer of titanium oxide and carbon black by reacting the outwardly diffused titanium atom with the oxygen gas to generate titanium oxide.
By heat-treating the titanium base material on which the mixed layer was formed in a vacuum atmosphere, TiO _x (x is 1.3 to 1.7 in the formula) was formed on the surface of the mixed layer. A process for producing titanium oxide and a method for manufacturing a separator for a fuel cell. The surface of a titanium base material made of titanium or a titanium alloy is coated with a mixed layer made of titanium oxide and carbon black.
A separator for a fuel cell, wherein the surface of the mixed layer _{is coated with titanium oxide of TiO x} (x is 1.3 to 1.7 in the formula).

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