JP5806099B2 - Surface treatment method for fuel cell separator - Google Patents

Description

  The present invention relates to a surface treatment method for a polymer electrolyte fuel cell separator having excellent corrosion resistance.

In recent years, from the viewpoint of global environmental conservation, development of fuel cells that are excellent in power generation efficiency and do not emit CO ₂ has been underway. This fuel cell generates electricity by electrochemical reaction from H ₂ and O ₂ , and its basic structure is an electrolyte membrane (ion exchange membrane), two electrodes (fuel electrode and air electrode), O ₂ (air ) And an H ₂ diffusion layer and two separators. And according to the type of electrolyte membrane used, it is classified into phosphoric acid fuel cell, molten carbonate fuel cell, solid oxide fuel cell, alkaline fuel cell, solid polymer fuel cell, etc. Development is underway.

Among these fuel cells, the polymer electrolyte fuel cell is compared with other fuel cells.
(a) The power generation temperature is about 80 ° C, and power can be generated at a significantly lower temperature.
(b) The fuel cell body can be reduced in weight and size.
(c) It has advantages such as a short rise time, high fuel efficiency and high power density. Therefore, solid polymer fuel cells are expected to be used as power sources for electric vehicles, stationary generators for home use or business use, and portable small generators. It is.

Here, the solid polymer fuel cell is one that takes out electricity from H ₂ and O ₂ through a polymer membrane. As shown in FIG. 1, gas diffusion layers 2 and 3 (for example, carbon paper) The membrane-electrode assembly 1 is sandwiched between the separators 4 and 5, and this is used as a single component (so-called single cell) to generate an electromotive force between the separator 4 and the separator 5.

  The membrane-electrode assembly 1 is called MEA (Membrance-Electrode Assembly), and the polymer film and an electrode material such as carbon black carrying a platinum catalyst are integrated on the front and back surfaces of the polymer film. The thickness is several tens of μm to several hundreds of μm. The gas diffusion layers 2 and 3 are often integrated with the membrane-electrode assembly 1 to form a single cell.

  Furthermore, when a polymer electrolyte fuel cell is put into practical use, it is common to use a single unit cell as described above by connecting several tens to several hundreds in series to form a fuel cell stack. is there.

In addition, the separators 4 and 5 include
(A) In addition to serving as a partition wall that separates single cells,
(B) a conductor that carries the generated electrons,
(C) Air flow path, hydrogen flow path through which O ₂ (air) and H ₂ flow,
(D) A function as a discharge path for discharging generated water and gas is required.
Therefore, it is necessary to use a separator having excellent durability and electrical conductivity for the polymer electrolyte fuel cell.

  Here, regarding durability, about 5000 hours are assumed when used as a power source for mounting an electric vehicle, and about 40,000 hours when used as a stationary generator for home use. is assumed. Therefore, the separator is required to have corrosion resistance that can withstand long-time power generation in order to ensure the durability described above. This is because proton conductivity of the electrolyte membrane decreases when metal ions are eluted by corrosion.

  On the other hand, regarding electrical conductivity, it is desired that the contact resistance between the separator and the gas diffusion layer is as low as possible. This is because as the contact resistance between the separator and the gas diffusion layer increases, the power generation efficiency of the polymer electrolyte fuel cell decreases. Therefore, the smaller the contact resistance between the separator and the gas diffusion layer, the better the power generation characteristics.

  To date, as the separators 4 and 5, solid polymer fuel cells using graphite have been put into practical use. The separators 4 and 5 made of graphite have an advantage that they have a relatively low contact resistance and do not corrode. However, since it is vulnerable to impact, it is difficult to reduce the size, and the processing cost for forming the air channel 6 and the hydrogen channel 7 is high. Therefore, these disadvantages of the separators 4 and 5 made of graphite hinder the spread of solid polymer fuel cells.

  Therefore, an attempt is made to apply a metal material instead of graphite as a material for the separators 4 and 5. In particular, from the viewpoint of improving durability, various studies have been made toward the practical application of separators 4 and 5 made of a stainless steel plate or a plate made of titanium or a titanium alloy.

  For example, Patent Document 1 discloses a technique in which a metal that easily forms a passive film such as stainless steel or a titanium alloy is used as a separator. However, the formation of a passive film leads to an increase in contact resistance, leading to a decrease in power generation efficiency. For this reason, it has been pointed out that these metal materials have problems to be improved such as a contact resistance larger than that of the graphite material.

  Patent Document 2 discloses a technique for reducing contact resistance and ensuring high output by performing gold plating on the surface of a metal separator such as an austenitic stainless steel plate (SUS304). However, it is difficult to prevent the occurrence of pinholes with thin gold plating, and conversely, the problem of cost remains with thick gold plating.

Japanese Patent Laid-Open No. 8-180883 Japanese Patent Laid-Open No. 10-228914

  An object of the present invention is to solve the above-mentioned various problems and to provide a surface treatment method for obtaining a polymer electrolyte fuel cell separator that is excellent in durability under an actual use environment.

In order to achieve the above object, the inventors have studied various surface treatment methods for a stainless steel plate as a material for a separator for a polymer electrolyte fuel cell. The elucidation process of the present invention will be described below.
First, it was found that after the Ni strike treatment of 0.5 μm was performed on the surface of the stainless steel plate, the Ni ₃ Sn ₂ layer was formed on the upper layer to a thickness of about 8 μm, thereby improving the durability in the environment where the separator was used. Here, the Ni strike treatment is performed in order to ensure adhesion of the upper Ni ₃ Sn ₂ layer.

  However, in the above-described forming process, for example, when a stainless steel plate having a plate thickness of 50 μm is used, a coating process of about 17 μm is required on both sides, resulting in an increase rate of the plate thickness of about 34%. When this is used for a fuel cell in which hundreds of cells are stacked, the overall thickness is extremely large, and when it is used in an automobile or the like, the mounting space becomes excessive and the mounting weight increases. .

In order to solve the above problem, the inventors tried to reduce the thickness of the Ni ₃ Sn ₂ layer. However, when the Ni ₃ Sn ₂ layer is simply thinned, the amount of defects that pass from the Ni ₃ Sn ₂ layer to the substrate increases. As a result, it was found that the Ni strike intermediate layer having poor corrosion resistance was corroded through the above-described defects, so that the upper Ni ₃ Sn ₂ layer was peeled off.

Next, the inventors tried to form the Ni ₃ Sn ₂ layer directly on the stainless steel plate without forming the Ni strike intermediate layer. However, in the case of a low Cr stainless steel sheet, although the adhesion of the Ni ₃ Sn ₂ layer can be ensured, a problem that the low Cr stainless steel sheet itself corrodes due to a defect connected to the substrate generated by thinning.

On the other hand, if the Ni ₃ Sn ₂ layer is formed directly on the high Cr stainless steel plate without forming the Ni strike intermediate layer, the adhesion of the Ni ₃ Sn ₂ layer is poor and it will peel off from the plate surface. Problem has occurred.

In response to the above examination, the inventors examined the surface treatment method itself that can improve the adhesion of the Ni ₃ Sn ₂ layer to the high Cr stainless steel sheet.
As a result, it was found that the reason why the adhesion of the Ni ₃ Sn ₂ layer was poor in the high Cr stainless steel sheet was the effect of the passive film having a high Cr content present on the surface of the high Cr stainless steel sheet.

Therefore, the inventors have further studied by considering that the adhesion of the Ni ₃ Sn ₂ layer can be secured by reducing the Cr content only on the surface of the high Cr stainless steel sheet. As a result, the base surface of the high Cr stainless steel sheet is subjected to anodic electrolytic treatment that causes a Cr transpassive dissolution reaction, so that the base surface is effectively reduced in Cr, and the Ni ₃ Sn ₂ layer directly formed on the base surface It was found that a high Cr stainless steel sheet capable of preventing peeling can be obtained.

The present invention has been made based on the above knowledge, and the gist configuration is as follows.
1. After subjecting the substrate surface of the stainless steel plate containing 21% by mass or more of Cr to an anodic electrolytic treatment that causes a Cr transpassive dissolution reaction, the Ni ₃ Sn ₂ layer is immediately formed by plating . A surface treatment method for a fuel cell separator, characterized in that the plate thickness increase rate is 1 to 20% .

2. 2. The surface treatment method for a fuel cell separator as described in 1 above, wherein the anodic electrolysis is an anodic electrolysis in an aqueous sodium sulfate solution.

3. The anodic electrolysis treatment in the aqueous sodium sulfate solution was conducted at a temperature of 60 to 80 ° C., a current density of 3 to 7 A / dm ² and an electrolysis time of 30 to 120 in a sodium sulfate aqueous solution having a concentration of 0.5 to 2 mol / L. 3. The surface treatment method for a fuel cell separator as described in 2 above, wherein the method is carried out under each condition of seconds.

  According to the present invention, since a fuel cell separator having excellent corrosion resistance can be produced effectively and thinly, it is possible to obtain a polymer electrolyte fuel cell which is compact and low in weight and further excellent in durability. it can.

It is a schematic diagram which shows the basic structure of a fuel cell. It is the graph which showed the thin film X-ray diffraction pattern of the plating film.

Hereinafter, the present invention will be specifically described.
In the present invention, the stainless steel used as the base material needs to be a high Cr stainless steel containing 21 mass% or more of Cr. This is because, in a low Cr stainless steel having a Cr content of less than 21% by mass, the low Cr stainless steel having poor corrosion resistance is corroded through defects connected to the substrate generated by thinning the Ni ₃ Sn ₂ layer.

  As the stainless steel used as the base material, for example, SUS447J1 (Cr: 30% by mass), SUS445J1 (Cr: 22% by mass), or the like can be preferably used. In particular, SUS447J1 containing about 30% by mass of Cr is particularly suitable as a separator base material for a polymer electrolyte fuel cell used in an environment where strict corrosion resistance is required because of its high corrosion resistance.

In the present invention, it is important to reduce only the base surface of the high Cr stainless steel sheet by subjecting the high Cr stainless steel sheet containing 21% by mass or more of Cr to anodic electrolytic treatment that causes a Cr transpassive dissolution reaction. It is. That is, by reducing the Cr only on the surface of the high Cr stainless steel plate, it is possible to obtain a high Cr stainless steel plate that can prevent the Ni ₃ Sn ₂ layer from peeling off. In addition, the present invention can be applied to a so-called high Cr stainless steel plate processed product obtained by subjecting a high Cr stainless steel plate to normal processing for a fuel cell separator, such as the high Cr stainless steel plate. The effect is obtained.

Here, the surface of the Cr stainless steel sheet is effectively reduced in Cr by performing an anodic electrolysis treatment in which Cr transpassive dissolution reaction occurs, due to the transpassive dissolution reaction of the chromium passive film shown below. This is thought to be due to the dechromation reaction.
_{Cr 2 O 3 + 5H 2 O} → 2CrO 4 2- + 10H + + 6e -

  Here, as the anodic electrolysis treatment in which Cr perpassive dissolution reaction occurs, it is only necessary to be able to perform anodic electrolysis by perpassive dissolution reaction, and the type and pH of the aqueous solution are not particularly limited, but a suitable example As sodium sulfate.

In addition, the conditions for the anodic electrolysis treatment may be set in the range of 60 to 80 ° C., current density of 3 to 7 A / dm ² , and electrolysis time of 30 to 120 seconds when using an aqueous sodium sulfate solution. preferable. This is because, in each of the above ranges, a hyperpassive dissolution reaction occurs effectively.

  By satisfying each of the processing conditions described above, the base surface of the stainless steel plate becomes a low Cr layer having a Cr content reduced by about 30% or more compared to the base material. Preferably, the Cr reduction rate is 50% or more.

In the present invention, it is necessary to form a Ni ₃ Sn ₂ layer immediately after performing an anodic electrolysis treatment that causes a Cr transpassive dissolution reaction on the substrate, without performing an intermediate layer formation treatment. By adopting such a procedure, it is possible to obtain a polymer electrolyte fuel cell separator having excellent corrosion resistance without the presence of a Ni strike intermediate layer having poor corrosion resistance.

In order to form a Ni ₃ Sn ₂ layer on the surface of an anodic electrolyzed substrate that causes a Cr transpassive dissolution reaction, electroplating in a plating bath adjusted to an appropriate composition by a conventionally known plating method Can be applied. The specific plating bath composition is as follows: NiCl ₂ · 2H ₂ O: 0.15 mol / L, SnCl ₂ · 2H ₂ O: 0.15 mol / L, K ₂ P ₂ O ₇ : 0.45 mol / L, Glycine: 0.15 An example is mol / L.
The thickness of the Ni ₃ Sn ₂ layer is preferably in the range of about 1 to 20% in terms of the increase rate of the thickness of the stainless steel plate from the viewpoint of ensuring corrosion resistance and mounting properties when the fuel cell is stacked. . The thickness of the Ni ₃ Sn ₂ layer can be adjusted by the residence time in the plating bath, that is, the electrolysis (plating) time.

FIG. 2 shows a thin film X-ray diffraction pattern of a plating film produced under the following plating bath composition and plating conditions. X-ray diffraction was measured using Cu-Kα rays at an incident angle of 5 degrees.
As is apparent from the figure, a film made of Ni ₃ Sn ₂ is formed on the surface.
<Plating bath composition>
NiCl ₂・ 2H ₂ O ： 0.15mol / L
SnCl ₂ · 2H ₂ O: 0.15 mol / L
K ₂ P ₂ O ₇ : 0.45 mol / L
Glycine: 0.15mol / L
<Plating conditions>
-PH: 8.1, temperature: 60 ° C, current density: 5A / dm ² , electrolysis (plating) time: 120 seconds

  In the present invention, manufacturing methods such as materials, equipment used, operating conditions, etc., other than those defined above, may be in accordance with conventional methods of surface treatment methods for fuel cell separators.

[Example 1]
Plate thickness: 0.05mm stainless steel plate, using SUS447J1 (Cr: 30% by mass), SUS445J1 (Cr: 22% by mass) and SUS430 (Cr: 16% by mass) as the base material. The anode electrolytic treatment was performed at 1 mol / L, temperature: 70 ° C., current density: 5 A / dm ² , and electrolysis time: 60 seconds. Next, a sample in which a Ni ₃ Sn ₂ layer having a thickness of 1 to 8 μm was formed on the substrate surface of the base material with the following plating bath composition and plating conditions without forming the Ni strike intermediate layer was prepared. A characteristic evaluation test was conducted. The thickness of the plating layer was controlled by examining the relationship between the electrolysis (plating) time and thickness in advance.
Table 1 shows the evaluation results of the obtained characteristics.
<Plating bath composition>
NiCl ₂・ 2H ₂ O ： 0.15mol / L
SnCl ₂ · 2H ₂ O: 0.15 mol / L
K ₂ P ₂ O ₇ : 0.45 mol / L
Glycine: 0.15mol / L
<Plating conditions>
-PH: 8.1, temperature: 60 ° C, current density: 5A / dm ²

1) Plating adhesion The cellophane tape (registered trademark) ("CT24", manufactured by Nichiban Co., Ltd.) was used for adhesion with the abdomen of the finger and then peeled off.
○: No plating peeling ×: Plating peeling

2) Plate thickness increase rate The plate thickness increase rate was calculated by the following formula.
Thickness increase rate (%) = [{(single-side coating film thickness) × 2} / stainless steel plate thickness] × 100
Moreover, the following criteria evaluated as the compactness at the time of a fuel cell stack.
○: Plate thickness increase rate 20% or less △: Plate thickness increase rate> 20%

3) Corrosion resistance The sample is immersed in a sulfuric acid aqueous solution at a temperature of 80 ° C and a pH of 3, and 0 to 1.2 V (vs.SHE) at a sweep rate of 200 mV / min using saturated KCl-Ag / AgCl as a reference electrode. The cyclic voltammogram (potential-current curve) of 5 cycles was applied, and the stability in the separator environment was evaluated by the current density value at 0.9 V when the voltage rises in the fifth cycle. The smaller the current density, the more the separator usage environment It is said that it has excellent corrosion resistance at the bottom.
The value of the current density at the fifth cycle of the voltage rising time of 0.9V ◎: Current density 2 .mu.A / cm ² or less ○: a current density of 2 super ~5μA / cm ² or less ×: current density 5 .mu.A / cm ² than

From the same table, high-Cr stainless steel sheets of SUS447J1 (Cr: 30% by mass) and SUS445J1 (Cr: 22% by mass), with a Ni ₃ Sn ₂ layer thickness in the range of 1 to 5 μm, are plated. It was found that the properties, the plate thickness increase rate, and the corrosion resistance were excellent. Further, it can be seen that when a low Cr stainless steel plate of SUS430 (Cr: 16% by mass) is used, although the plating adhesion is excellent, it is impossible to achieve both the plate thickness increase rate and the corrosion resistance.

  Sample No. The base surface of the stainless steel plate 1 was measured by Auger electron spectroscopy, and as a result, the Cr content was reduced by about 80% compared to the base material.

[Example 2]
As a comparative example, a stainless steel plate with a plate thickness of 0.05 mm, and the base material is hydrochloric acid using SUS447J1 (Cr: 30% by mass), SUS445J1 (Cr: 22% by mass) and SUS430 (Cr: 16% by mass). After pickling treatment at a concentration of 3 mol / L, temperature: 70 ° C., time: 60 seconds, after forming a Ni strike intermediate layer of 0.5 μm with the following plating bath composition and plating conditions, the same conditions as in Example 1 A sample in which a Ni ₃ Sn ₂ layer was formed with a thickness of 1 to 8 μm was prepared, and the characteristics were evaluated under the same conditions as in Example 1.
Table 2 shows the evaluation results of the obtained characteristics.
<Plating bath composition>
・ NiCl ₂・ 2H ₂ O: 1 mol / L, hydrochloric acid: 1 mol / L
<Plating conditions>
・ Temperature: 25 ℃, Current density: 10A / dm ²

  According to the table, when the Ni strike intermediate layer is formed, plating is possible regardless of the steel type of SUS447J1 (Cr: 30% by mass), SUS445J1 (Cr: 22% by mass), or SUS430 (Cr: 16% by mass). Although it is excellent in adhesiveness, it turns out that the plate | board thickness increase rate and corrosion resistance are not able to be made compatible.

Example 3
As a comparative example, a stainless steel plate with a thickness of 0.05 mm is further used. The base material is SUS447J1 (Cr: 30% by mass), SUS445J1 (Cr: 22% by mass) and SUS430 (Cr: 16% by mass). After pickling treatment with hydrochloric acid concentration: 3 mol / L, temperature: 70 ° C., time: 60 seconds, Ni strike layer was not formed, and Ni ₃ Sn ₂ layer was directly formed under the same conditions as in Example 1. Samples formed in a thickness range of 1 to 8 μm were prepared, and the characteristics were evaluated under the same conditions as in Example 1.
The evaluation results of the obtained characteristics are shown in Table 3.

From the same table, after pickling treatment, when Ni ₃ Sn ₂ layer is formed directly without Ni strike intermediate layer, high Cr stainless steel of SUS447J1 (Cr: 30 mass%) and SUS445J1 (Cr: 22 mass%) Since the steel plate is not subjected to anodic electrolytic treatment that causes a Cr transpassive dissolution reaction, plating adhesion is poor. On the other hand, if a low-Cr stainless steel plate of SUS430 (Cr: 16% by mass) is used, plating Although it is excellent in adhesiveness, it turns out that a plate | board thickness increase rate and corrosion resistance cannot be made compatible. In addition, although the example of the stainless steel plate was shown in the said Example, even if it processes by the present invention after processing a stainless steel plate, it has confirmed that an equivalent effect is acquired.
Further, in this example, a sodium sulfate aqueous solution suitable for anodic electrolysis by Cr perpassive dissolution reaction of the chromium passive film was used, but the present invention only needs to be able to perform anodic electrolysis by perpassive dissolution reaction. As described above, the type and pH of the aqueous solution are not limited thereto.

By following the present invention, a stainless steel plate or a processed product thereof containing Cr 21% by weight or more, after being subjected to anode electrolytic treatment Cr excessive passivation dissolution reaction occurs, without an intermediate layer formation process, immediately Ni ₃ Sn By forming the _two layers, it is possible to provide a light and compact separator material for a polymer electrolyte fuel cell that is excellent in plating adhesion and corrosion resistance, has a low thickness increase rate, and is compact.

1 membrane - electrode assembly 2 gas diffusion layers 4 and 5 separator 6 O ₂ (air) passage 7 hydrogen flow path

Claims (3)

After subjecting the substrate surface of the stainless steel plate containing 21% by mass or more of Cr to an anodic electrolytic treatment that causes a Cr transpassive dissolution reaction, the Ni ₃ Sn ₂ layer is immediately formed by plating . A surface treatment method for a fuel cell separator, characterized in that the plate thickness increase rate is 1 to 20% .   The surface treatment method for a fuel cell separator according to claim 1, wherein the anodic electrolysis is an anodic electrolysis in an aqueous sodium sulfate solution. The anodic electrolysis treatment in the aqueous sodium sulfate solution was conducted at a temperature of 60 to 80 ° C., a current density of 3 to 7 A / dm ² and an electrolysis time of 30 to 120 in a sodium sulfate aqueous solution having a concentration of 0.5 to 2 mol / L. 3. The method for surface treatment of a fuel cell separator according to claim 2, wherein the method is performed under each condition of seconds .

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