JP2003077486A - Fuel cell separator and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for fuel cell that is of low cost and has excellent conductivity, and is superior in durability and corrosion resistance, and a method of manufacturing the separator simply and easily. SOLUTION: The separator, which is used for a solid polymer fuel cell, is made by a base material by forming an iron based material having a carbon content of 0.1 wt.% or more and 1 wt.% or less and by forming a carbide coated on the surface of the base material by TRD treatment. The separator is made which has a good property demonstrating the characteristics of the carbide coating by TRD treatment. And the base material is formed by press molding and then it undergoes the TRD treatment, and, then, distortion is eliminated. Since the thermal distortion due to the TRD treatment is eliminated, a separator having a stable configuration is manufactured simply and easily.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a separator used in a polymer electrolyte fuel cell and a method for manufacturing the same.

[0002]

2. Description of the Related Art Separator used in a polymer electrolyte fuel cell has a function of forming a passage for fuel gas and oxygen gas (air) while being provided on both sides of a reaction electrode to separate adjacent cells. , Which fulfills the function of collecting current from the reaction electrode. Therefore, good gas tightness and electrical conductivity are required. In order to meet this requirement, conventionally, a solid polymer fuel cell separator is preferably used in which a gas passage is formed by cutting a densely fired carbon black plate. However, such a separator has poor productivity,
The disadvantage is that the manufacturing cost is high, and there is an increasing demand for inexpensive separators.

As a separator that can be manufactured in large quantities at low cost, a separator using a metal plate-shaped material as a base material has begun to be proposed, and in connection with this, the following techniques exist. For example, it is described in JP-A No. 11-162479, and is a technique relating to a separator in which a metal coating in which conductive ceramics are dispersed is formed on the surface of a base material such as stainless steel or aluminum. Kaihei 11
JP-A-219713 and JP-A-2000-21420, which relate to a separator in which a surface of a base material such as stainless steel is coated with an oxide-based, nitride-based, or carbide-based conductive ceramics. It is a technology.

[0004]

In all of the above techniques, a conductive coating film is formed on the surface of the base material, but the coating film is formed by a thin film forming method such as so-called PVD, CVD, sputtering or the like. However, the adhesion of the coating film to the substrate is poor and there is a problem in corrosion resistance. Further, it cannot be said that the corrosion resistance is good because the film thickness is thin. Fuel cell separators are often exposed to oxidizing atmospheres,
A fuel cell using a separator having poor corrosion resistance has poor durability. The present invention has been made in view of the above problems, is inexpensive, has excellent conductivity, and
An object of the present invention is to provide a fuel cell separator having excellent durability and corrosion resistance, and to provide a method for easily manufacturing the separator.

[0005]

A fuel cell separator of the present invention is a separator used in a polymer electrolyte fuel cell, and is made of an iron-based material having a carbon content of 0.1 wt% or more and 1 wt% or less. And a TR on the surface of the substrate
And a carbide coating formed by the D treatment.

TRD processing is performed by Thermo-Reactive Depositi
This is a surface treatment method called on and Deffusion Process, and in principle, it utilizes precipitation and diffusion by a thermal reaction based on a small free energy of formation. For example, by immersing the treated base material (base material) in a molten salt, the carbide and nitride forming elements added to the salt bath are thermally combined with C and N in the base material, A carbide layer on the surface of the substrate,
It is a surface treatment method for forming a nitride layer.

When a carbide coating is formed by this TRD treatment, the carbide coating is extremely hard, thermally and chemically stable, dense, and excellent in adhesion to the substrate. Further, it exhibits sufficient corrosion resistance even under severe conditions such as various corrosion resistant aqueous solutions of hydrochloric acid, sulfuric acid, nitric acid, sodium hydroxide, seawater, etc. and high temperature chlorine gas atmosphere. Furthermore, the conductivity of the coating is as good as that of carbon. A fuel cell separator having a carbide coating formed by TRD treatment having such characteristics is inexpensive, has excellent conductivity, and has excellent corrosion resistance and corrosion resistance.

The method for producing the fuel cell separator of the present invention is one aspect of the method for producing the fuel cell separator of the present invention, wherein the carbon content is 0.1 wt% or more and 1 wt% or less. A base material forming step of forming a base material by press-forming a thin plate made of an iron-based material using a mold.
TRD treatment is performed on the molded base material to form a carbide coating film on the surface of the base material, and a strain removing step of removing thermal strain generated in the base material by the TRD treatment. Characterize.

The separator used in the polymer electrolyte fuel cell forms a gas flow path as described above. Therefore, when an iron-based thin plate is used as a base material, the surface of the thin plate is formed into an uneven shape by press molding. After such press molding, TRD processing is performed. TRD
The treatment, as will be shown later, is a treatment that can be performed simply and quickly, so that the productivity and the cost required for the treatment are also low, but since it is a treatment at a relatively high temperature, thermal strain occurs, The base material itself may be deformed. In such a case, it suffices to remove the generated thermal strain, and the removal method itself is simple and quick, as will be described later in detail, so that it does not cause significant inhibition of productivity and increase in cost. . Therefore, according to the present manufacturing method, it is possible to manufacture a separator having the above characteristics and having a stable shape at high productivity and at low cost.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the fuel cell separator and the method for producing the same according to the present invention will be described in detail below. In addition, the embodiment to be described is one embodiment, and the fuel cell separator and the manufacturing method thereof according to the present invention are not limited to the following embodiments. The embodiment can be implemented in various forms including modifications and improvements that can be made by those skilled in the art.

<Separator> The fuel cell separator of the present invention comprises a base material and a carbide coating formed on the surface thereof. The base material has a carbon content of 0.1 wt% or more and 1 wt% or more.
It consists of the following iron-based materials. An inexpensive separator can be realized by using an iron-based material. Carbon content is 0.
If less than 1%, T
The formation of a carbide coating by RD treatment cannot be fully expected. Further, if it exceeds 1 wt%, it is hard and brittle, and when forming irregularities for forming gas passages on the surface by press molding as described later, cracks and the like are likely to occur, impairing the airtightness of the separator. there's a possibility that. As the iron-based material, various steel materials such as carbon steel, alloy steel such as tool steel and high speed steel, and stainless steel can be used.

The shape of the base material is not particularly limited, but since the fuel cell is used by stacking a thin electrolyte-electrode assembly, it is preferably a thin plate-shaped material. Further, since it is necessary to form a gas passage between the electrode and the electrode, it is desirable that the surface is uneven and the protrusion is in contact with the electrode. The shape of the unevenness is not particularly limited, and may be, for example, one in which protrusions are uniformly formed over the front surface of the surface, or one having a plurality of protrusions with a long number of threads (how many (Like a groove is formed)
May be When the thin plate-shaped material is used, these irregularities may be formed by press molding.

In the fuel cell separator of the present invention,
A carbide coating is formed on the surface of the base material by TRD treatment. The carbide coating is made of vanadium carbide (VC), niobium carbide (NbC), chromium carbide (Cr ₇ C ₃ + Cr).
₂₃ C ₆ ), titanium carbide (TiC), etc. may be used alone or in combination of two or more. Among these, vanadium carbide (VC) is a carbide of the present separator because of its good conductivity, excellent adhesion to a substrate, easy formation of a relatively thick coating, and excellent corrosion resistance. It is particularly suitable as a coating.

The thickness of the carbide coating is not particularly limited, but is preferably 3 μm or more. 3 μm
If it is less than the above range, the durability and corrosion resistance may be poor. In order to obtain more sufficient durability, it is desirable to form a film having a thickness of 5 μm or more. If the thickness is too thick, the effect of improving the durability and the like cannot be obtained compared with the increase of the processing cost, so the thickness is preferably 20 μm or less.

The specific process of TRD processing is not particularly limited. It may be performed by a molten salt method, a fluidized bed furnace method, a powder method or the like. For example, in the molten salt method, the base material may be immersed in a molten salt bath. In this case, the molten salt bath is made of borax (Na ₂ B ₄ O ₇ ) as a bath base or an additive containing a chloride such as barium chloride or a metal that becomes a carbide. The additive can be selected according to the carbide to be produced, and for example, in the case of vanadium carbide, Fe-V (ferrovanadium) may be used. Similarly, for example, in the case of niobium carbide, Fe-Nb, in the case of chromium carbide, Cr, and in the case of titanium carbide, Fe-T.
It is sufficient to select each i. The temperature of the molten salt bath is 70
The temperature may be 0 to 1200 ° C., and the immersion time may be several tens of minutes to 20 hours depending on the thickness of the carbide coating. The TRD treatment by the molten salt method is completed by washing the deposited salt attached to the substrate taken out from the bath. In addition, T
The number of times the RD process is performed is not particularly limited,
When it is performed a plurality of times, the cost is slightly increased, but the durability and corrosion resistance of the obtained carbide coating are improved.

Further, in the fluidized bed furnace method, for example, the substrate may be placed in a fluidized bed furnace made of mixed powder prepared as follows. The mixed powder is a refractory powder such as alumina for suppressing solidification during flowing, and an activator powder such as a halide that promotes a carbide forming reaction, and a powdered metal-containing metal to be added. It can be prepared by mixing with an agent. Then, a fluidizing gas is introduced into the mixed powder to form a fluidized bed, and the base material is put therein. As the fluidizing gas, an inert gas such as argon gas or a non-oxidizing gas such as nitrogen gas may be used. The temperature and processing time of the fluidized bed furnace may be the same as those in the molten salt method.

Further, in the powder method, the substrate may be covered with a treatment agent containing the activator powder and the powdered additive and heated. In this case, the treatment is performed in an atmosphere of an inert gas such as argon gas or a non-oxidizing gas such as nitrogen gas. The treatment temperature and the treatment time may be the same as in the molten salt method.

<Production Method of Separator> The production method of the present invention is one mode of the method of producing the separator of the present invention, and includes a substrate forming step, a film forming step, and a strain removing step. .

In the base material forming step, the carbon content is 0.1 wt.
% Or more and 1 wt% or less, a thin plate made of an iron-based material is press-molded using a mold. As the iron-based material, those mentioned above may be used. In this step, the unevenness for forming the above-mentioned gas passage is mainly formed. The specific method of press molding is not particularly limited, for example, by sandwiching the thin plate by a press die consisting of an upper die and a lower die, and pressing with a predetermined pressing force, The unevenness having a shape corresponding to the mold is formed. Additional molding such as trimming of the outer shape may be performed in this step.

The coating film forming step is a step of forming a carbide coating film on the surface of the base material, and is performed by performing the above-mentioned TRD treatment. The specific process of TRD processing may follow the process described above. It may be accompanied by a pretreatment process such as purification of the surface of the base material performed before the TRD treatment. Since the TRD treatment is a treatment at a relatively high temperature as described above, the base material itself is distorted. Since press molding is performed in the base material forming step, processing strain due to the press molding remains in the base material. The residual stress caused by this is released by the TRD treatment at a high temperature, so that the base material itself is deformed. So-called thermal strain occurs. Such thermal strain cannot secure the flatness over the entire surface of the separator (specifically, the coplanarity of the convex portion or the concave portion), which hinders the uniform contact between the electrode and the separator, and causes the current collecting function of the separator to be impaired. Become.

The strain removing step to be performed next is a step for removing the thermal strain and ensuring the flatness of the separator. The specific method is not particularly limited.
For example, the shape of the base material may be corrected by applying pressure again using equipment such as a press. In that case, in order to remove strain easily and efficiently, it is desirable to reheat the substrate. The reheating temperature is 200 to 200 in the atmosphere from the viewpoint of preventing the oxidation of carbides.
It may be about 450 ° C. In a non-oxidizing atmosphere such as N ₂ gas, the temperature can be raised to about 800 ° C. The higher the reheating temperature, the better the effect of shape correction. In this case, it is desirable to raise the temperature of the press die to the reheating temperature in advance. Further, the base material may be heated at the same time as the pressure is applied by using a press die whose temperature is raised. By heating the press die in advance, it can be held at a high temperature for a long time, and a good shape correction effect can be obtained. As the press die, the die used in the previous base material forming step can be used. The mold is originally a mold for molding the base material into a desired shape, and by using the mold, the base material can be easily modified and molded into the desired shape. The separator manufactured through the above steps,
It is used for assembly of a polymer electrolyte fuel cell together with an electrolyte-electrode assembly and the like. When the desired flatness can be obtained by the material of the base material, the conditions of the TRD processing, the design of a special jig for fixing the base material in the TRD processing, etc., without performing the strain removing step, The TRD-treated separator may be used for assembling the fuel cell.

[0022]

Example A separator was actually manufactured based on the above embodiment, a solid polymer fuel cell was manufactured using the separator, and the characteristics of the fuel cell were evaluated. In addition, the characteristics of the separator itself were compared with those of other separators having different configurations. These will be described below.

<Production of Separator> FIG. 1 shows the shape of the produced separator (strictly speaking, the base material). 1A is a partial perspective view, and FIG. 1B is a partial cross-sectional view. The steel plate used is a 0.2 mm thick stainless steel plate (SUS4
40A equivalent product), and the carbon content is 0.6 to 0.7.
It is in the range of wt%. First, in a base material forming step, this steel sheet was press-formed by a press die as schematically shown in FIG.

Then, the molded substrate was subjected to TRD treatment by a molten salt method in a film forming step to form a vanadium carbide film on the surface. The salt bath temperature in the TRD treatment was about 1000 ° C., and the immersion time was 5 hours. The thickness of the produced vanadium carbide coating is
It was about 8 μm. Next, the TRD-treated base material is
In the strain removal process, reheat to a temperature of about 400 ° C,
Similarly, sandwich it in the mold shown in FIG.
The thermal strain due to the TRD treatment was removed by re-pressurizing. Thus, the separator was completed.

By the way, FIG. 3 shows the result of investigation on the flatness of the substrate after each step. The sled height shown in the figure is
It is a value at a length of 40 mm. After the base material molding process, that is, before the TRD process, the variation in the sled height is
0.03 to 0.3 mm, with an average value of about 0.05
It was mm. By the TRD process, the variation is 0.
05 to 1 mm, the average value increased to 0.4 mm,
By removing the strain, the variation was reduced to 0 to 0.05 mm, and the average value was reduced to 0.03 mm. It can be confirmed that the flatness sufficient for use was obtained by the strain removal step.

<Fabrication of Fuel Cell> A one-cell solid polymer fuel cell was fabricated using the above separator. FIG. 4 schematically shows the fuel cell. FIG. 4A shows the arrangement of the respective constituent elements before stacking, and FIG. 4B shows
The state where they were laminated is shown. Electrolyte-electrode assembly 1
Is composed of an electrolyte membrane 1a and an air electrode 1b and a fuel electrode 1c sandwiching the electrolyte membrane 1a. The electrolyte membrane 1a has a Nafion
Air electrode 1 using N115 (trade name: manufactured by DuPont)
b and the fuel electrode 1c have P as a catalyst on the electrolyte membrane side.
A carbon cloth supporting t was used. Separator 2
Is a separator that has been subjected to the TRD treatment on both the air electrode side and the fuel electrode side. The separator 2 is housed in a resin separator frame 3 so that fuel gas and air are efficiently supplied to the electrode surface. Further, a water cooling frame 4 made of metal is arranged integrally with the separator 2 on the fuel electrode side. By the way, the electrode area is 13 cm ² (36
mm × 36 mm).

<Characteristic Evaluation of Fuel Cell>
Discharge was performed using hydrogen gas as the fuel gas. The discharge conditions are
Maintain the cell temperature at 80 ℃ (85 ℃ for the humidifier on the fuel side and 70 ℃ for the humidifier on the air side) and use 2.0 × 1 fuel gas.
Flow at a flow rate of 0.5 L / min (0.01 L / min when not discharging) under a pressure of 0 ⁵ Pa, and the air is
A flow rate of 1.5 L / min (0.1 L / min when not discharging) was applied under a pressure of 2.0 × 10 ⁵ Pa.

FIG. 5 shows the discharge characteristic curve of this battery.
Incidentally, FIG. 5 also shows a discharge characteristic curve under the same conditions of a fuel cell using a conventional product made of a carbon black plate subjected to a cutting process as a separator. As can be seen from the discharge characteristic curve shown in FIG. 5, the fuel cell using the iron-based material separator having the carbide coating formed by the TRD treatment can be compared with the fuel cell using the conventional carbon separator. It can be confirmed that the characteristics are comparable to each other.

Further, a continuous discharge of 10 hours a day was repeated every day (discharging was stopped for 14 hours), and a total of 500 hours of discharge was performed. FIG. 6 shows the battery voltage and the battery resistance. By the way, as the battery resistance, a value obtained by superposing a 1 kHz AC current (amplitude 1 A) on the discharge current and using the AC component of the voltage response is adopted. As can be seen from FIG. 6, even when the cell was operated for up to 500 hours, the deterioration was not observed at all, and the fuel cell using the separator of the iron-based material on which the carbide coating by TRD treatment was formed was durable. It can be confirmed that there is no problem in sex.

<Characteristics of Separator> The contact resistance with the carbon cloth constituting the electrode of the separator was measured.
To measure the contact resistance, as shown in FIG. 7, two sample materials having the same structure as the above separator were used, and two carbon cloths that were the same as the carbon cloths that compose the electrodes were sandwiched between them, and the outside Sandwiched between two copper blocks with silver paste on the contact surface, 10kg / c on both sides
A load of m ² was applied, an alternating current of 1 A and 1 kHz was applied between both blocks, and the voltage between both blocks was measured. In addition, the measurement was performed immediately after production (before the exposure test), and after the exposure test for 24 hours and the exposure test for 96 hours. The exposure test simulated the environment of the fuel cell and was exposed to a saturated steam atmosphere at 60 ° C.

FIG. 8 shows the TRD measured under the above conditions.
The contact resistance of the iron-based material separator formed by forming the carbide coating by the treatment is shown. For comparison, FIG. 8 also shows the contact resistances of separators having other configurations. The separators of other configurations have a thickness of 1
0 μm Sn plating, same shape SUS3
There are three types of 16 L base materials, which are not subjected to any surface treatment, and those which are conventionally used carbon black plates.

As can be seen from this figure, the separator having the carbide coating by the TRD treatment shows a contact resistance comparable to that of the conventional carbon separator. The separator made of SUS316L, which is said to have excellent weather resistance, has a high contact resistance, which is one of the causes of increasing the internal resistance of the battery, and it can be confirmed by an exposure test that the contact resistance increases, which is favorable. It is hard to say that it is a characteristic separator. Further, it can be said that the Sn-plated product has a low contact resistance and shows good characteristics, but, for example, corrosion is severe in an atmosphere such as the next test, that is, in an acidic atmosphere, and durability is a problem. Will be left.

Next, the corrosion resistance in an acidic atmosphere was evaluated by a corrosion test of immersing in an aqueous sulfuric acid solution. The sample material is a separator made of SUS304, which is formed into the same shape as the separator having the carbide (VC) coating by the TRD treatment and is not subjected to any surface treatment. The sulfuric acid aqueous solution to be dipped is of two types, that is, the one with a concentration of 10% and the one with a concentration of 50%. ,
It was soaked for 25 hours and 50 hours. The results of this corrosion test are shown in FIG. It can be confirmed that the separator made of SUS304 exhibits a state of general corrosion even in a corrosion test under any condition, and cannot be a separator excellent in corrosion resistance. On the other hand, the separator having the carbide (VC) coating formed by the TRD treatment exhibited local corrosion under some conditions, but generally good results were obtained.
Therefore, it can be said that this separator has excellent corrosion resistance.

[0034]

The fuel cell separator of the present invention comprises a base material made of an iron-based material and a carbide coating formed on the surface thereof by TRD treatment. The TRD-treated carbide coating is extremely hard, thermally and chemically stable, dense, and excellent in adhesion to the substrate. Further, even under severe conditions such as the presence of various corrosion resistant aqueous solutions such as acids, etc., sufficient corrosion resistance is exhibited and the conductivity of the coating film is also good. Combined with the use of an inexpensive iron-based material for the base material, the separator of the present invention is inexpensive as a separator used in a polymer electrolyte fuel cell, has excellent conductivity, and has corrosion resistance and corrosion resistance. It will be excellent.

The manufacturing method of the present invention is one mode of the method of manufacturing the separator of the present invention, and includes a step of removing thermal strain due to the TRD treatment after the TRD treatment. Therefore, according to the manufacturing method of the present invention,
A separator with a stable shape can be manufactured with high productivity and at low cost.

[Brief description of drawings]

FIG. 1 shows the shape of a separator manufactured in an example.

FIG. 2 schematically shows how press molding is performed in the substrate forming step.

FIG. 3 shows the flatness of each step of the separator.

FIG. 4 schematically shows the polymer electrolyte fuel cell produced in the example.

FIG. 5 shows a discharge characteristic curve of the manufactured fuel cell.

FIG. 6 shows the repeated discharge performance of the produced fuel cell.

FIG. 7 schematically shows a device configuration for measuring a contact resistance between a separator and a carbon cloth that constitutes an electrode.

FIG. 8 shows the measurement results of the contact resistance between the separator and the carbon cloth forming the electrode.

FIG. 9 shows the results of a corrosion test performed on the separator.

[Explanation of symbols]

1: Electrolyte-electrode assembly 1a: Electrolyte film 1b: Air electrode 1c: Fuel electrode 2: Separator 3: Separator frame 4: Water cooling frame

Continued front page    (72) Inventor Toshihiko Tani             Aichi Prefecture Nagachite Town Aichi District             Local 1 Toyota Central Research Institute Co., Ltd. F-term (reference) 5H026 AA06 BB01 BB02 BB04 CX04                       EE02 EE05 EE11 HH00 HH03

Claims (6)

[Claims] 1. A separator used in a polymer electrolyte fuel cell, comprising a base material made of an iron-based material having a carbon content of 0.1 wt% or more and 1 wt% or less, and a TRD treatment on the surface of the base material. And a carbide coating formed by the fuel cell separator. 2. The fuel cell separator according to claim 1, wherein the carbide coating has a thickness of 3 μm or more. 3. The fuel cell separator according to claim 1, wherein the carbide coating is a vanadium carbide coating. 4. A method for manufacturing a separator used in a polymer electrolyte fuel cell, wherein a thin plate made of an iron-based material having a carbon content of 0.1 wt% or more and 1 wt% or less is pressed using a mold. A base material forming step of forming and forming a base material; a film forming step of applying a TRD treatment to the formed base material to form a carbide coating on the surface of the base material; A method for manufacturing a fuel cell separator, comprising: 5. The method for manufacturing a fuel cell separator according to claim 4, wherein the strain removing step includes a step of reheating the base material and press-molding the base material. 6. The method for manufacturing a fuel cell separator according to claim 5, wherein the press forming in the strain removing step is performed using the mold used in the base material forming step.