WO2003034525A1 - Solid polymer electrolyte type fuel cell-use separator and production method therefor - Google Patents

Abstract

A stainless steel-made separator, especially, a solid polymer electrolyte type fuel cell-use separator having both high corrosion resistance and high conductivity, and a production method therefor. The solid polymer electrolyte type fuel cell-use separator is formed by coating the surface of stainless steel having a general corrosion index, GI=[Cr%]+3.6[Ni%]+4.7[Mo%]+11.5[Cu%], of at least 90 with a conductive compound such as metal nitride and metal-dispersed diamond-like carbon. When forming a coating, ion bombarding is conducted on the surface of the above high-corrosion-resistance stainless steel as a pre-treatment, and then a conductive compound is applied by ion plating. High corrosion resistance and high conductivity are ensured by this surface coating at coated portions, and high corrosion resistance is supported by the passive coating of stainless steel itself at non-coated portions (defective portions such as pin holes).

Description

 Description Separator for solid polymer electrolyte fuel cell and method for producing the same Technical field

 The present invention relates to a separator used for a fuel cell, and more particularly to a solid polymer electrolyte type fuel cell separator and a method for producing the same. Background technology

 In recent years, solid polymer electrolyte fuel cells generally have high power generation efficiency and

Since it has the advantage of not emitting SOx and NOx, it has been spotlighted as a small-scale fuel cell for home and automobile use, and is expected to be used for stationary power generation and automotive power generation, respectively. Conventionally, this type of solid polymer electrolyte fuel cell generally has the following structure in brief.

 In a solid polymer electrolyte fuel cell, the solid polymer electrolyte membrane has a thickness of about 50 / m, and electrodes are bonded to both sides of the electrolyte membrane. The electrode is basically made of a porous platinum material.However, platinum fine particles are attached to carbon black fine particles, mixed with a small amount of Teflon fine particles and other polymers, and thinly applied to carbon fiber paper. It has a two-layer structure of porous graphite and carbon fiber paper. A single cell of a solid polymer electrolyte fuel cell has a structure in which the graphite porous body side of this electrode is overlaid on both sides of the electrolyte membrane and thermocompression bonded to form a single unit. A single unit is formed by arranging graphite separators on the carbon fiber paper side. As such, fuel cells are typically used as a stack of multiple units.

Conventionally, in a solid polymer electrolyte fuel cell, since a fuel gas and an oxidizing gas flow on both sides of an electrode, a grooved separator formed by grooving graphite is usually used as the separator. I have. The graphite grooved separator has a structure in which the fuel gas and the oxidizing gas are not mixed with each other as a flow path substrate with the flow path formed on both sides of the electrode. The graphite grooved separator serves as a partition for each cell, preventing the fuel gas and the oxidizing gas from being mixed as described above, and It also serves as a path for the current generated in each cell.

 However, graphite separators are fragile due to their material, and thus have a problem of lacking impact resistance, especially when used in automotive fuel cells. In addition, there was a problem that extra processing costs were required, such as the need for cutting, to add graphite grooves, and the fuel cell cost increased accordingly.

 Therefore, metal separators have recently been proposed in place of such graphite separators. If the separator is made of metal, it will not only have high impact resistance, but also does not require cutting, and can be formed by pressing, so processing costs are low and volume efficiency is good. This is because there is an advantage that compacting can be achieved.

 By the way, in a solid polymer electrolyte fuel cell, the solid polymer electrolyte membrane has the same main chain as Teflon and has a structure having a sulfone group at the end, and the sulfone group is ionized, and the proton is ion-conductive. Shows sex. For this reason, the solid polymer electrolyte membrane is acidic, and the electrodes and the separator are required to have corrosion resistance to acid. In particular, when a metal material having low corrosion resistance is used as the separator, the eluted metal ions react with the electrolyte membrane, and the ionic conductivity of the membrane is impaired, resulting in a decrease in power generation capacity. Therefore, a metal separator must first have high corrosion resistance.

 Next, the separator forms a boundary between the cells as described above, and not only prevents the fuel gas and the oxidizing gas from being mixed, but also serves as a passage for the current generated in each cell. Therefore, it is necessary to have sufficient conductivity.

 Therefore, a stainless steel separator has been attracting attention as a metal separator which is likely to satisfy the requirements for high corrosion resistance and good conductivity.

However, stainless steel is indeed known for its excellent corrosion resistance, which is maintained by the passivation film formed on the surface, while the passivation film is insulating, Impedes conductivity. Therefore, conventionally, when a stainless steel separator is provided, from the viewpoint of securing the function of the separator, corrosion resistance is ensured by the passivation film on the surface, but conductivity is hindered by the passivation film. There is a problem that it is necessary to eliminate the adverse effects. Accordingly, an object of the present invention is to solve the above-mentioned conventional problems and to provide a separator for a solid polymer electrolyte fuel cell having particularly high corrosion resistance and good conductivity among stainless steel separators. Is to do. Disclosure of the invention

 The present inventors have conducted intensive studies and as a result, as a means for solving the above-mentioned technical problems, first, a stainless steel is subjected to a surface treatment such as ion plating to thereby achieve high corrosion resistance and good conductivity. We have found a configuration that forms a film of a conductive compound that has both. In the present invention, the term “film” simply refers to a coating layer formed on the surface of the underlying stainless steel by a surface treatment such as ion plating, unless otherwise specified.

 In addition, since stainless steel mainly contains hydroxides such as chromium, which are the components contained in the passivation film, the higher the content of chromium and molybdenum, the higher the passivation film on the surface, and the higher the corrosion resistance of the passivation film. It is known that a film formed by ion plating or the like inevitably has a defect such as a pinhole. These defects are generated due to various causes such as the generation of dust and the like during the film formation process and the generation of fine cracks due to the grain boundaries of the film. It is impossible to produce a film free of defects. If there is a defect such as a pinhole, no matter how excellent the corrosion resistance of the film itself, the underlying metal ions elute from a pinhole or the like without a film, which reacts with the electrolyte membrane and reacts with the electrolyte membrane. Results in the inhibition of ionic conductivity. Therefore, we focused on the fact that it is necessary for the underlying stainless steel that at least the underlying metal ions do not elute even if defects such as pinholes occur in the coating. It has been found that the required high corrosion-resistant stainless steel is used in the present invention, and the present invention having the following configuration has been completed.

That is, the separator for a solid polymer electrolyte fuel cell of the present invention, when expressed in terms of% by mass, is [Cr%] + 3.6 [Ni%] + 4.7 [Mo%] + 11.5 [Cu %] Of stainless steel having a corrosion resistance of 90 or more. If the surface of stainless steel is coated with a conductive compound in this way, Since there is no passivation film between the film and the underlying metal in the coated part, good conductivity is ensured, but with regard to corrosion resistance, the coated part also has high corrosion resistance due to the corrosion resistance of the film itself. The coating (the part with defects such as pinholes) is secured by the passivation film of the underlying high corrosion-resistant stainless steel itself. As a result, a solid polymer electrolyte fuel cell separator having both high corrosion resistance and good conductivity, which has not existed in the past, is provided, and the object of the present invention is achieved. In addition, since the present invention is a stainless steel separator in the first place, the material is not as brittle as a conventional graphite separator in terms of material, and is required for an automobile fuel cell or the like that requires impact resistance. Can also be widely used. Further, there is no need to perform cutting for forming a groove for the flow path, and the molding can be performed easily and inexpensively by pressing. As a result, a low-cost fuel cell can be provided. In addition, compactness can be achieved because of good volumetric efficiency.

 In the separator for a solid polymer electrolyte fuel cell of the present invention, the conductive compound may be any compound having both high corrosion resistance and good conductivity. Examples of the conductive compound include metals such as chromium nitride and titanium nitride. Preferably, it is a nitride.

Further, in the present invention, the conductive compound is preferably a metal-dispersed diamond-like carbon, which is a substance obtained by dispersing a metal in diamond-like carbon to impart conductivity. This is because the metal-dispersed diamond-like carbon film has excellent acid resistance, but is a material obtained by adding conductivity to diamond-like carbon, which is an insulator, by adding a metal. Therefore, in the present invention, as described above, the conductive compounds coated on the surface of stainless steel are metal nitride and metal-dispersed diamond-like carbon, so that the coated portion can more effectively ensure high corrosion resistance and Good conductivity can be ensured. Now, in the present invention, when forming a conductive compound film on the surface of stainless steel, the high corrosion-resistant stainless steel having an overall corrosion resistance of 90 or more is used as a base metal, and ion bombing is performed on the surface of the stainless steel. Is preferably performed as a pretreatment, and then the conductive compound is coated by ion plating. Therefore, by this manufacturing method, the passivation film existing on the surface of stainless steel is removed in advance, the adhesion between the film and the base metal is improved, and high corrosion resistance and And a film having good conductivity can be effectively formed on the stainless steel surface. BRIEF DESCRIPTION OF THE FIGURES

 FIG. 1 is a chart showing the relationship between the overall corrosion resistance coefficient and corrosion weight loss under predetermined conditions for stainless steels having various component compositions. BEST MODE FOR CARRYING OUT THE INVENTION

 Hereinafter, embodiments of the present invention will be described in more detail with reference to the drawings. The solid polymer electrolyte fuel cell separator of the present invention has a structure in which a high corrosion-resistant stainless steel is used as a base metal, and the surface of the stainless steel is coated with a conductive compound.

 That is, the present invention forms a conductive compound film having both high corrosion resistance and good conductivity by subjecting the surface of high corrosion resistance stainless steel to surface treatment by ion plating described later. The conductive compound is preferably a metal nitride such as chromium nitride, titanium nitride, etc., having both high corrosion resistance and good conductivity. Such a conductive compound film may be a metal-coated material, for example, a two-layer structure of metal chromium and chromium nitride, or a metal titanium and titanium nitride film as a pretreatment of the nitride film coating. Having a two-layer structure is effective in further increasing the corrosion resistance.

 The coating of the conductive compound in the present invention is effective even if it is coated with metal-dispersed diamond-like carbon. Examples of the metal-dispersed diamond-like carbon film include a film in which metal tungsten is dispersed in a tungsten carbide film and a film in which metal tantalum is dispersed in a tantalum carbide film. Diamond-like carbon such as tungsten carbide and tantalum carbide has excellent acid resistance, but because it is an insulator, it imparts conductivity by the addition of metal.

 The thickness of the coating on the underlying stainless steel is not limited. However, from the viewpoint of ensuring corrosion resistance, it is preferably 2 to 3 or more.

Here, the results of examining specifically what kind of steel is effective as the base metal as the high corrosion-resistant stainless steel used in the present invention will be described. The present inventors have For stainless steel having a seed composition, the weight loss at 80 ° C. in 10% sulfuric acid and 20% sulfuric acid was measured. The results are summarized in terms of overall corrosion resistance (hereinafter referred to as “GI”). As shown in Fig. 1, a clear correlation was observed between GI and corrosion weight loss. Here, GI is an index obtained by using GI = [Cr%] + 3.6 [Ni%] + 4.7 [Mo%] + 11.5 [Cu%] using mass%. From this relationship, it was found that if the GI was 90 or more, sufficient corrosion resistance could be obtained.

Thus, solid polymer electrolyte fuel cell separator of the present invention, as the base metal, indicating the mass _{^{0/0, [Cr%]}} + 3. 6 [Ni%] + 4. 7 [Mo%] + 1 A high corrosion resistant stainless steel having a total corrosion resistance of 1.5 [Cu%] of 90 or more is selected, and the surface of the stainless steel is coated with the conductive compound.

 Stainless steel with a GI of 90 or more includes, for example, SUS317 and SUS310S in JIS grades, and NTK30AC (C≤0.07, Cr: 19.00 to 21.00, Ni: 32.00 to 38.00, Mo: 2.00 to 3.00) as other commercial grades. , Cu: 3.00-4.00, Nb: 8xC% -10xC%), NT 30A (C≤0.020, Cr: 19.00-21.00, Ni: 28.00-30.00, Mo: 2.00-3.00, Cu: 3.00-

4.00, Mn: 2.50 to 3.50), NTK22A (C≤0.020, Cr: 19.00 to 21.00, Ni: 21.00 to 23.00, Mo: 1.75 to 2.75, Cu: 1.75 to 2.75, Mn: 2.50 to 3.50) and the like. However, the present invention is not limited to the above-listed steel types as the base high corrosion-resistant stainless steel.

 Next, a method of forming a coating of a conductive compound by applying a surface treatment to the underlying high corrosion-resistant stainless steel will be described.

 In the present invention, the above-mentioned high corrosion-resistant stainless steel having a GI of 90 or more is used as a base metal, and the surface of the stainless steel is preferably subjected to ion bombering as a pretreatment. Coating conductive metal compounds such as metal dispersed diamond-like carbon.

In the ion plating process, an ion bombering process is performed before the process in order to improve film adhesion. In the ion bombing process, argon ions are accelerated to collide with the surface of the workpiece to remove impurities and oxide films on the surface, thereby cleaning the surface and coating the substrate continuously. It improves the adhesion between the film formed by the treatment and the underlying metal. The passivation film existing on the surface of the stainless steel is removed in advance by this ion bombering treatment. Therefore, if ion bombing is performed as a pretreatment prior to the ion plating process, the passivation film does not exist between the film and the base metal in the coating portion where the film has been formed, and as a result, the stainless steel itself And the conductivity of the film is ensured. Regarding the corrosion resistance, the coating part is also assured of high corrosion resistance by the corrosion resistance of the film itself.

 On the other hand, pinholes and other defects (uncovered areas) where no film is formed expose the underlying stainless steel directly to the outside, while the stainless steel passive film comes into contact with atmospheric moisture and oxygen. As a result, it is formed naturally, so that corrosion resistance is ensured. By the above film forming method, a separator for a polymer electrolyte fuel cell having both high corrosion resistance and good conductivity is produced, and the object of the present invention is achieved.

 As a method for forming a film, a method that can achieve the object of the present invention, that is, a method for forming a film having both high corrosion resistance and good conductivity on the surface of stainless steel, is not particularly limited to the above method. None, but ion plating is one of the best methods. Further, a more preferable result can be obtained by adopting a configuration including a step of previously removing a passivation film of stainless steel called an ion bomber ring as a pretreatment.

 【Example】

 Next, the present invention will be described in further detail and more specifically based on the following examples. Naturally, the present invention is not limited to these examples.

 (1) Sample preparation

Chromium nitride was coated on a stainless steel plate (thickness 0.2 mm, steel type NTK30AC) using a multi-arc plating system. In that case, first, as a pretreatment, while flowing argon gas ^{5~ 1 0 x 1 0- 2 Pa} , I was subjected to ion bombardment ring by applying a voltage 4 0 0 V. Next, using chromium as the electrode, a nitrogen gas was flowed into the chamber at a pressure of 0.5 Pa, and an arc was generated on the electrode and one wall of the chamber at a voltage of 200 V and a current of 8 OA. While rotating the plate, chromium nitride was coated at 5 μπι (hereinafter referred to as “CrN coating material”). After ion bombing of the stainless steel plate with the same equipment as above, metal chromium was coated at 2 μπι in an argon gas atmosphere, and then a chromium nitride layer was formed at 3 μππ in a nitrogen atmosphere (hereinafter, referred to as "Cr / CrN coating material").

 After ion bombing the stainless steel plate using a magnetron sputtering device, a film in which metal tungsten was dispersed in tungsten carbide was formed in a mixed gas of argon and acetylene using a tungsten target (hereinafter, referred to as “metal”). "W-DLC coating material").

 (2) Evaluation of characteristics

 Conductivity and corrosion resistance of the CrN coating, CrZCrN coating and W-DLC coating were evaluated as follows.

The conductivity was evaluated by measuring the contact electric resistance of the film with a contact electric resistance measuring instrument [Yamazaki-type SQ meter manufactured by Yamazaki Seiki Laboratory Co., Ltd.]. A graphite electrode was brought into contact with the sample surface at a contact pressure of 2 O KgZcm ² , and the contact electric resistance was measured when scanning by 1 O mm. As a result, the CrN coating material was 0.355 Ω, the Cr / CrN coating material was 0.285 Ω, and the W-DLC coating material was 0.593 Ω, which was pure stainless steel. It is much smaller than Ω, and close to 0.158 Ω pure gold.

The corrosion resistance was evaluated by immersing in 5% sulfuric acid at 80 ° C for 148 hours and measuring the elution amount on the film surface. Elution amount, CrN coating material is ^{0. 2 5 mg / m 2} / h, Cr / CrN coating material is ^{0. 2 2 mg / m 2} / h, W-DLC coating material 0. 3 8 mg / m ² / h, and a small value.

 Next, after the above three types of film-formed stainless steel plates were pressed into a grooved structure, they were assembled as a separator in a single-cell fuel cell, and a power generation test was performed. When the polymer electrolyte membrane and the integrated electrode were sandwiched by this separator and oxidizing gas and hydrogen gas were supplied to both ends, an initial voltage of 0.7 V was obtained. Furthermore, after continuous operation for 500 hours, an output of about 0.67 V was obtained in each case. This is only a 5% voltage drop. Industrial applicability

As described above, the present invention is a solid polymer having both high corrosion resistance and good conductivity among stainless steel separators used for fuel cells for automobiles and homes. It is useful as a separator for an electrolyte fuel cell, and is suitable for producing such a separator for a solid polymer electrolyte fuel cell.

Claims

The scope of the claims In terms of 1-mass ⁰ , [Cr%] + 3.6 [Νί%] +4.7 [Μο%] + 11.5 [Cu%] A separator for a solid polymer electrolyte fuel cell, comprising a surface coated with a conductive compound.  2. The separator for a polymer electrolyte fuel cell according to claim 1, wherein the conductive compound is a metal nitride.  3. The separator for a solid polymer electrolyte fuel cell according to claim 1, wherein the conductive compound is a metal-dispersed diamond-like carbon.  4. The stainless steel is used as a base metal, the surface of the stainless steel is subjected to ion bombering as a pretreatment, and then the conductive compound is coated by ion plating. The method for producing a separator for a solid polymer electrolyte fuel cell according to 1, 2, or 3.