JP2003092117A - Separator for fuel cell and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for a fuel cell preventing the peeling off or the crack of a gold plating layer on the surface of a stainless steel plate, and remarkably enhancing corrosion resistance and durability, and to provide the manufacturing method of the separator for the fuel cell. SOLUTION: Voids are formed on the surface of the stainless steel plate by intergranular corrosion and the gold plating layer if formed in an embedded state on the voids. When an average crystalline grain size on the surface of the stainless steel plate is represented by L (μm) and the thickness of the gold plating layer is by d (μm), a limit value of bending R generating peeling or the crack in the gold plating layer is made smaller by satisfying the relation of 0.2<=4/d/L<=80.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a separator constituting a polymer electrolyte fuel cell and a method for manufacturing the same.

[0002]

2. Description of the Related Art In a polymer electrolyte fuel cell, a laminated body in which separators are laminated on both sides of a flat plate-like electrode structure (MEA: Membrane Electrode Assembly) constitutes one unit.
A plurality of units are stacked to form a fuel cell stack. The electrode structure has a three-layer structure in which an electrolyte membrane made of an ion exchange resin or the like is sandwiched between a pair of gas diffusion electrodes forming a positive electrode (cathode) and a negative electrode (anode). The gas diffusion electrode has a gas diffusion layer formed outside the electrode catalyst layer that is in contact with the electrolyte membrane. Further, the separator is laminated so as to be in contact with the gas diffusion electrode of the electrode structure, and a gas flow path or a refrigerant flow path for allowing gas to flow is formed between the separator and the gas diffusion electrode. According to such a fuel cell, for example, hydrogen gas, which is a fuel, is caused to flow in the gas flow passage facing the gas diffusion electrode on the negative electrode side, and oxygen or air is oxidized in the gas flow passage facing the gas diffusion electrode on the positive electrode side. When a volatile gas is flowed, an electrochemical reaction occurs and electricity is generated.

The separator supplies electrons generated by the catalytic reaction of hydrogen gas on the negative electrode side to an external circuit,
It is necessary to have a function of sending electrons from the external circuit to the positive electrode side. Therefore, a conductive material made of graphite-based material or metal-based material is used for the separator. Particularly, the metal-based material has excellent mechanical strength and can be made lighter and more compact by thinning it. It is said to be advantageous in that Examples of the metal separator include those obtained by pressing a thin plate made of a metal material having high corrosion resistance such as stainless steel or titanium alloy to have an uneven cross section.

By the way, as environmental factors in which such a separator is placed, there are the following three factors. Temperature: Since the movable temperature of the fuel cell is from room temperature to about 180 ° C., the separator is also exposed to this temperature range.

PH: In the fuel cell, oxygen reacts with hydrogen to produce water. This water is generated in the form of water vapor, but when the temperature drops on the gas flow path formed in the separator, it adheres to the separator as droplets. Also,
When the amount of the attached water increases, the water stays between the electrode structure and the separator. Then, the water attached to the electrode structure easily contacts the electrolyte membrane. Therefore, liberation of substituents in the electrolyte membrane occurs, hydrogen ions are generated in the attached water, and the pH of the attached water decreases. A sulfone group is generally used as a substituent of a fuel cell, and the attached water exhibits acidity such as sulfuric acid.

Explaining the above substituents, in the fuel cell, hydrogen ions produced from hydrogen on the catalyst on the hydrogen gas supply side (anode side) are transferred to the oxidizing gas supply side (cathode side). , Supplying hydrogen ions to the cathode catalyst. Then, power is continuously generated by using the action of reacting with the oxidizing gas and generating water on the cathode catalyst as a driving force. Therefore, the electrolyte membrane of the fuel cell must be a cation conductive type electrolyte membrane capable of moving hydrogen ions from the anode side to the cathode side. Therefore, the side chain of the electrolyte membrane molecule must have a bonding group capable of bonding with hydrogen ions. In the fuel cell, the above function is satisfied by arranging an acid type substituent that bonds a part of the molecule of the electrolyte membrane with hydrogen ion. Since this substituent is an acid type, it generates an acid when liberated from the electrolyte membrane. Generally, in order to improve the hydrogen ion transfer efficiency of the electrolyte membrane, a strong acid type having a high binding force with hydrogen ions is used for this substituent, and therefore, it is liberated to generate an acid, and the generated acid is low. It becomes pH.

Potential: Separators are placed on the fuel gas side and the oxidizing gas side, and serve as the positive electrode and the negative electrode of the cell, respectively. An electromotive force generated by the reaction is generated as a potential difference between these two separators. Generally, in a fuel cell using hydrogen as the fuel gas and oxygen as the oxidizing gas, the maximum potential difference caused by electromotive force is 1.2 V for the following reason.
Occurs to some extent. That is, the electromotive force obtained by the chemical reaction of producing water from hydrogen and oxygen is theoretically about 1.2 V in the operating temperature range of the fuel cell, and the same is true in actual power generation. A voltage of about 1 to 1.2 V is generated. When an austenitic stainless steel plate having high corrosion resistance is used as the separator, if the electromotive force exceeds about 0.9 V, the elution rate of metal ions increases, which causes corrosion.

As described above, a separator for a fuel cell is a condition in which each of the factors of temperature, pH and potential is extremely corrosive. Therefore, when a metal separator is used, it is a material having high corrosion resistance. (For example, SUS316
Even L) is easily corroded. Therefore, the metallic separator is required to have extremely high corrosion resistance in the operating environment of the fuel cell. Further, in addition to this, the metal separator is easy to press for forming a gas flow path and a refrigerant flow path with an uneven cross-section, and from the viewpoint of preventing a decrease in generated voltage, it is different from other members. Very low contact resistance is required. Furthermore, since several hundreds of separators are used in one fuel cell stack in some cases, low cost is also required.

Therefore, it is desirable that the surface of a stainless steel plate that can be easily pressed is plated with a highly corrosion-resistant metal as a fuel cell separator. Here, the corrosion resistance of stainless steel SUS316L, Cu, Ag, Pt, Au was compared by measuring the corrosion current density under conditions of a temperature of 90 ° C., a sulfuric acid solution of pH 3 and 1.2 V, respectively. 156μA / cm
² , Cu: 98 μA / cm ² , Ag: 38 μA / c
m ² , Pt: 18 μA / cm ² , Au: ² μA / c
It was m ² . In order to ensure practical durability as a fuel cell, it is desirable that the corrosion current density be 10 μA / cm ² or less, and therefore it is understood that the element that satisfies the condition is gold. Therefore, a material such as a stainless steel plate plated with gold is promising as a separator for a fuel cell.

[0010]

However, when the stainless steel is gold-plated by the usual method, the gold plating is only physically attached to the stainless steel, and therefore the adhesion is not good. It can be said that it is not so expensive. Therefore, when a separator having an uneven cross section and a bent portion having a very small R is formed by press working, peeling of the gold plating is likely to occur due to insufficient adhesion. When peeling occurs, the contact resistance between the gold plating and the stainless steel plate that is the base material increases, and low contact resistance cannot be satisfied.
When the peeled gold plating is dropped or the plated layer is cracked by press working, the exposed stainless steel plate is easily corroded, so that the corrosion resistance required for the fuel cell cannot be satisfied.

Therefore, an object of the present invention is to provide a fuel cell separator capable of preventing peeling and cracking of a gold plating layer applied to the surface of a stainless steel plate and greatly improving corrosion resistance and durability, and a method for producing the same. There is.

[0012]

The fuel cell separator of the present invention is a fuel cell separator in which a gold coating layer is formed on the surface of a stainless steel plate, and the surface of the stainless steel plate has voids due to intergranular corrosion. Are formed, and a gold coating layer is formed so as to be embedded in the voids. As the stainless steel plate, for example, an austenitic stainless steel plate is used, and the crystal grain boundaries on the surface in that case are austenite crystal grain boundaries.

According to the fuel cell separator of the present invention, the corrosion resistance is improved by forming the gold coating layer on the surface of the stainless steel plate. The gold coating layer is the surface of the stainless steel plate. It is in a state of being embedded in the void formed by intergranular corrosion. Therefore, the gold coating layer firmly adheres to the surface of the stainless steel plate due to the physical anchor effect. Therefore, even if pressed to form a gas flow path or the like, peeling or cracking of the gold coating layer at the bent portion is prevented, and excellent durability is exhibited.

Next, the method for producing a fuel cell separator of the present invention is a method for producing the above separator preferably, in which the surface of a stainless steel plate is subjected to intergranular corrosion treatment and the treated surface is coated with gold. After that, press molding is performed. As a method of corroding the surface of the stainless steel sheet at the grain boundary, for example, a chemical etching treatment can be adopted, and voids are formed on the surface of the stainless steel sheet by the grain boundary corrosion. As a method for coating the surface with gold, a general plating method can be adopted. When the gold plating layer is formed, a part of the gold plating layer is embedded in the void formed by the intergranular corrosion.
Thereby, the separator of the present invention can be manufactured.

In the present invention, since the voids formed by intergranular corrosion increase and the adhesion of the gold coating layer increases, the crystal grain size on the surface of the stainless steel sheet is small and there are many grain boundaries. Is desirable. However, for example, when the gold coating layer is a gold plating layer, a pit-like defect is likely to occur in the gold plating layer on the grain boundary. Therefore, if there are too many voids, the defects increase and the density increases. When the press working is performed in such a state, the defects are interconnected by cracks, which causes cracks in the gold plating layer.
For this reason, it is desirable that the crystal grain size on the surface of the stainless steel plate be within a certain range. Further, the anchor effect described above largely depends on the thickness of the gold coating layer, and the anchor effect is often obtained when the gold coating layer is appropriately thin. Therefore, by defining the anchor distance per unit volume of the gold coating layer, it is possible to define the optimum crystal grain size according to the gold coating layer. Therefore, as a result of various studies on this point, the present inventor has determined that the average crystal grain size on the surface of the stainless steel sheet is L (μ
m), assuming that the thickness of the gold coating layer is d (μm), if L and d satisfy 0.2 ≦ 4 / d / L ≦ 80, a sufficient anchor effect can be obtained. It was Therefore, in the present invention, this condition is a preferable mode.

[0016]

EXAMPLES Next, examples of the present invention will be described. Stainless steel having the composition shown in Table 1 (equivalent to SUS316L)
A square plate of 100 mm × 100 mm and a thickness of 0.2 mm, having an average crystal grain size L of 15 μm and 5
A large number of μm test pieces were prepared.

[0017]

[Table 1]

These test pieces were divided into three groups A, B and C.
The surfaces of the test pieces of each group were intergranularly corroded using different types of chemical etching solutions. There are the following three types of chemical etching solutions. -A group: 10% nitric acid, 4% hydrofluoric acid, 50 ° C bath-Group B: 20% nitric acid, 8% hydrofluoric acid, 50 ° C bath-C group: 25% nitric acid, 50% hydrochloric acid, 2 glycerin
5%

Next, gold plating layers having different thicknesses were applied to the surfaces of the test pieces for each group. The bath salt of the gold plating layer is gold cyanide K: 12 g / l, citric acid K: 125 g
/ L, EDTA Co salt: 3 g / l, bath salt temperature,
The thickness of the gold plating layer was adjusted by appropriately changing the current density and the time. After forming the gold plating layer, “4 / d / L” was obtained for each test piece, where the average crystal grain size on the surface was L (μm) and the thickness of the gold plating layer was d (μm).
Subsequently, each test piece was bent to examine the limit value of the bending R at which peeling or cracking occurred in the gold plating layer. The relationship between the above “4 / d / L” and the limit value of bending R for each group is shown in FIG.
Shown in.

According to FIG. 1, the average grain size on the surface is L
(Μm), when the thickness of the gold plating layer is d (μm), the value of “4 / d / L” is in the range of 0.2 ≦ 4 / d / L ≦ 80, and the bending R is small, around 100 μm. However, it is recognized that if the value deviates from this range, the limit value of the bending R is significantly increased. Therefore, the value of “4 / d / L” is 0.2 ≦ 4 / d / L
It was confirmed that in the range of ≤80, a separator in which peeling and cracking of the gold plating layer were prevented and strong durability was maintained for a long period of time could be obtained.

[0021]

As described above, according to the present invention,
Since voids are formed on the surface of the stainless steel plate which is the base material due to intergranular corrosion, and the gold coating layer is formed in a state of being embedded in the voids, bending R in which peeling or cracking occurs in the gold coating layer The limit value of is extremely small, whereby peeling or cracking of the gold coating layer is prevented, and corrosion resistance and durability are greatly improved.

[Brief description of drawings]

FIG. 1 shows a relational expression “4 / d / L” regarding an average crystal grain size L (μm) on a surface of a base stainless steel plate and a thickness d (μm) of a gold plating layer measured in an example of the present invention. ,
It is a diagram which shows the relationship with the limit value of bending R.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Masao Utsunomiya             1-4-1 Chuo Stock Market, Wako City, Saitama Prefecture             Inside Honda Research Laboratory F-term (reference) 4K024 AA11 AB01 BA04 BB28 DA07                       DB07 GA01                 4K057 WA05 WB02 WB12 WE02 WE30                       WF04 WK06 WN10                 5H026 AA06 BB02 BB04 BB10 EE02                       EE08 HH01 HH03

Claims (4)

[Claims] 1. A fuel cell separator in which a gold coating layer is formed on the surface of a stainless steel plate, wherein voids are formed on the surface of the stainless steel plate due to intergranular corrosion, and the state of being embedded in the voids. A separator for a fuel cell, wherein the gold coating layer is formed on the. 2. When the average crystal grain size on the surface of the stainless steel plate is L (μm) and the thickness of the gold coating layer is d (μm), L and d are 0.2 ≦ 4 / d /. The fuel cell separator according to claim 1, wherein L ≦ 80 is satisfied. 3. A method for producing a fuel cell separator, which comprises subjecting a surface of a stainless steel plate to intergranular corrosion treatment, coating the treated surface with gold, and then performing press molding. 4. When the average crystal grain size on the surface of the stainless steel plate is L (μm) and the thickness of the gold coating layer is d (μm), L and d are 0.2 ≦ 4 / d / L ≦ 80 is satisfied, The manufacturing method of the fuel cell separator of Claim 3 characterized by the above-mentioned.