JP2006302729A - Stainless steel separator for polymer electrolyte fuel cell and polymer electrolyte fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stainless steel separator for a polymer electrolyte fuel cell keeping high corrosion resistance while lowering surface contact resistance by forming a thin passive film having high concentration of Cr on the surface of ferritic stainless steel. <P>SOLUTION: Ferritic stainless steel containing 15-40 mass% Cr and less than 1 mass% Mo is used as a base material, and the whole surface of the base material is immersed in a nonoxidative acid solution to form a passive film which has a contact resistance to carbon paper of 10 mΩ cm<SP>2</SP>or less at a measuring pressure of 20 kgf/cm<SP>2</SP>. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a stainless steel separator incorporated in a polymer electrolyte fuel cell that can be operated at a low temperature and is easy to maintain.

  The polymer electrolyte fuel cell can operate at a low temperature of 100 ° C. or less, and has an advantage of starting up in a short time. Further, since each member has a simple structure made of a solid, it is not only easy to maintain, but can also be applied to applications where it is exposed to vibration and impact. In addition, the high power density is suitable for downsizing, high fuel efficiency, and low noise.

In order to take out the amount of electric power that can be put to practical use from a fuel cell with a very small amount of power generation per cell, it is necessary to stack a large number of cells with a unit of a solid polymer membrane sandwiched between separators. Since this separator is required to have good conductivity and low contact resistance, a graphite separator has been used conventionally. However, the graphite separator not only has a high manufacturing cost, but also easily breaks when excessive vibration or impact is applied. For this reason, in the application for mounting on automobiles that has been attracting attention recently, there is a high risk of cracking leading to breakage, and it is difficult to use. Therefore, for example, as seen in Patent Documents 1 and 2, application of stainless steel in place of graphite is being studied.
JP-A-9-157801 JP 2000-239806 A

Stainless steel has the advantages that it can be thinned due to its high strength and excellent ductility, and can be easily formed into a target shape by an inexpensive processing method such as press molding. In addition, the steel plate surface is covered with a passive film formed from oxides and hydroxides of Cr, Mo, Fe, etc., which are constituents of stainless steel, and the barrier effect of this passive film prevents corrosion of the base steel. It has the characteristic that it is.
Although the passive film is effective for improving the corrosion resistance, it exhibits semiconducting properties and is inferior in electrical conductivity as compared to the base steel. For this reason, when stainless steel with a normal passive film is used for the separator, the contact resistance with the electrode is large, the electric energy generated by the cell reaction is consumed as Joule heat, and the power generation efficiency of the fuel cell decreases. To do.

In order to apply stainless steel to the separator while utilizing excellent corrosion resistance, it is necessary to reduce the contact resistance of the stainless steel surface. As a means for reducing the contact resistance, the precious metal coating, the roughening of the stainless steel surface, and the like have been studied.
However, the expensive noble metal coating increases the cost of the fuel cell, and restricts the spread of the fuel cell from the economical aspect. Moreover, in the noble metal coating method, pinholes that cause pitting corrosion are easily formed in the film, so that strict product management is required. Thick plating forms a noble metal film without pinholes, but large consumption of expensive noble metals is a bottleneck in cost reduction.

When a stainless steel is roughened to reduce contact resistance, an alternating electrolysis method in a ferric chloride bath is employed. However, since it is electrolytic treatment, a large facility is required.
As another means for reducing the contact resistance, an acid dipping method can be mentioned. When stainless steel is immersed in an acid solution, the passive film dissolves and a new passive film is formed in the acid solution. This passive film is a very thin film and has a lower contact resistance than a naturally occurring passive film. However, when it is taken out from the acid solution and left in the atmosphere, it reacts with oxygen in the atmosphere and a passive film grows to increase the contact resistance. Further, the oxidation electrode side of the fuel cell is an acidic moist atmosphere with a low pH value. When stainless steel is exposed to such an atmosphere, the contact resistance further increases, which causes a decrease in the efficiency of the fuel cell.

  In the present invention, the tendency of contact resistance to increase when a stainless steel plate, which has been subjected to acid solution immersion treatment to reduce contact resistance, is allowed to stand in a cell environment of a fuel cell, varies greatly depending on the Cr concentration in the passive film. Based on this new knowledge, the formation of a passive film with a high Cr concentration and a thin film thickness on the surface of ferritic stainless steel maintains excellent corrosion resistance without using precious metal coating or roughening treatment. An object of the present invention is to provide a stainless steel separator that exhibits low surface contact resistance.

The stainless steel separator for a polymer electrolyte fuel cell of the present invention is made of a ferritic stainless steel containing Cr: 15 to 40% by mass and Mo: less than 1% by mass, and the entire surface thereof is non-oxidizing. It is characterized by being immersed in an acid solution.
The obtained stainless steel separator preferably has a contact resistance with carbon paper of 10 mΩ · cm ² or less at a measurement pressure of 20 kgf / cm ² . Further, it is preferable that the contact resistance with carbon paper after being left for 72 hours at 70 ° C. and 98% relative humidity is 25 mΩ · cm ² or less at a measurement pressure of 20 kgf / cm ² .
When such a separator is mounted on an actual machine, a polymer electrolyte fuel cell with high power generation efficiency can be obtained.

According to the present invention, a high Cr content stainless steel is formed by using a high Cr concentration ferritic stainless steel as a base material, and a new Cr-enriched passive film is formed by dipping in a non-oxidizing acid solution. A stainless steel separator that maintains low contact resistance even when exposed to an acidic wet atmosphere in a fuel cell for a long period of time while utilizing the high corrosion resistance inherent in steel can be obtained.
As a result, a fuel cell that is inexpensive, excellent in long-term corrosion resistance, and stable in power generation efficiency is constructed.

  The passive film formed on the surface of stainless steel is usually greatly affected by the surface finish in the final process when the stainless steel plate is manufactured. In particular, a passive film on a stainless steel plate that has been subjected to bright annealing finish, dull finish, or the like exhibits a large contact resistance. In addition, since the pickling finish is also washed with hydrofluoric acid, which is an oxidative acid, the thickness of the formed passive film is thick, and the surface contact resistance is that of the polymer electrolyte fuel cell. It is not of a level that can be used as a separator.

Accordingly, the present inventors have conducted various studies on a method for obtaining a passive film exhibiting a low contact resistance value. As a result, the present inventors have found that a stainless steel material is obtained by using a high Cr concentration ferritic stainless steel as a base material and immersing this stainless steel in a non-oxidizing acid solution.
Ferritic stainless steel with a high Cr content exhibits excellent corrosion resistance in an acidic humid atmosphere found in the environment of fuel cells, and there are few eluted metals such as Ni and Cu that adversely affect ion exchange membranes and catalyst electrodes. . Corrosion resistance under an acidic wet atmosphere is further improved by increasing the Cr concentration in the steel.

Moreover, as the stainless steel has a higher Cr concentration and better corrosion resistance, the thickness of the passive film formed in the acid solution is thinner, and the contact resistance is smaller than that of the stainless steel having a low Cr concentration. In addition, according to the knowledge of the present inventors, when re-passivation is performed by immersing a high Cr concentration ferritic stainless steel in a non-oxidizing acid solution such as hydrochloric acid or sulfuric acid, the concentration of Cr is moderately concentrated. A dynamic film is formed, which not only improves the corrosion resistance but also maintains the contact resistance at a low level.
Thereby, the material for separators which satisfies high corrosion resistance and low contact resistance is provided.

Stainless steel has higher corrosion resistance as the Cr concentration increases. Since the passive film formed by repassivation also becomes thin as the Cr concentration in the stainless steel increases, it is also effective in reducing the contact resistance. Furthermore, since the corrosion resistance is good, an increase in the thickness of the passive film due to oxidation of the steel substrate, that is, oxidation of Fe, is suppressed even under conditions of high temperature and high humidity, which are the in-cell environment of the fuel cell.
The reason why the contact resistance of the ferritic stainless steel containing a high concentration of Cr is maintained at a low level is considered as follows.

  When a ferritic stainless steel containing a high concentration of Cr is immersed in a non-oxidizing acid solution, the passive film possessed before immersion is completely dissolved, and a new passivation is obtained in the non-oxidizing acid solution. A film is formed. In addition, since dissolution of stainless steel in a non-oxidizing acid solution such as hydrochloric acid proceeds entirely on the surface of the stainless steel, the passive film before immersion is completely dissolved and hydrofluoric acid, which is an oxidizing acid. There is no increase in the thickness of the passive film due to excessive Cr concentration as seen in immersion. However, when stainless steel is melted, the dissolution rate of the steel components varies depending on the element. Since Cr has a slower dissolution rate than Fe, Cr is concentrated on the surface of the stainless steel during immersion in the acid solution. For this reason, at the end of the immersion treatment, a passive film having a thin film thickness is regenerated while maintaining a Cr-rich state as compared with the steel substrate. And the thin passive film containing a high concentration of Cr acts as a barrier that shields the base steel from the external environment. As a result, even when exposed to an acidic wet atmosphere in the fuel cell, the passive film is maintained in a thin state, and an increase in contact resistance is suppressed.

Next, each requirement of the present invention will be described in detail.
The stainless steel targeted by the present invention is a ferritic stainless steel containing 15 to 40% by mass of Cr.
Cr is a main element in securing the corrosion resistance of stainless steel, and the corrosion resistance improves as the content increases. Since the inside of the cell of the fuel cell is a highly corrosive environment that exhibits a low pH value due to acidic substances, 15 mass% or more of Cr is necessary. Furthermore, in order to maintain low contact resistance, it is preferable to make it contain 20 mass% or more. Since the workability decreases with increasing Cr content, the upper limit was set to 40% by mass.

In order to actually evaluate a stainless steel separator incorporated in a fuel cell, a separator having an overhang height of 0.4 mm and an overhang width of 2 mm was produced by press molding. Molding was possible when the Cr content was 40% by mass or less, but in the material containing 42% by mass of Cr, cracks occurred in the overhanging portion.
The passive film becomes thinner as the stainless steel has a higher Cr content. A thin passive film is advantageous in reducing contact resistance with the electrode when stainless steel is applied to a fuel cell separator. Moreover, stainless steel containing a high concentration of Cr maintains sufficient corrosion resistance even in the acidic environment of the fuel cell. That is, high Cr content ferritic stainless steel is excellent in both electrical conductivity and corrosion resistance, and thus can be said to be a suitable material for a fuel cell separator.

  Mo is an alloy component that contributes to the corrosion resistance of stainless steel together with Cr. The effect of improving pitting corrosion resistance can be obtained by adding Mo. On the other hand, excessive addition not only hardens the stainless steel and lowers the workability, but also leads to an increase in cost. Particularly for a separator that is forced to have a complicated shape, a decrease in workability leads to the occurrence of cracks and distortion, leading to a decrease in productivity. Since the internal environment of the polymer electrolyte fuel cell is not a salt damage environment, pitting corrosion is unlikely to occur. Therefore, in consideration of workability, the addition of Mo is suppressed as much as possible and is set to less than 1% by mass.

The stainless steel to which the present invention is directed contains C, N, Mn, Si, P, and S as components other than Cr and Mo as usual. If necessary, Ni, Cu, Ti, Nb, Al, V, etc. may be contained.
C and N reduce the workability and low-temperature toughness of ferritic stainless steel, so they should be reduced as much as possible. Preferably, the C and N contents are both limited to 0.02% by mass or less.

Since Mn is easily eluted even in a state of maintaining a passive state, it is preferably regulated to 0.6% by mass or less.
Since Si hardens stainless steel and reduces workability, it is preferably regulated to 0.5% by mass or less.

P is an effective component for improving the corrosion resistance in the internal environment of the fuel cell to which the separator is exposed, but if it is excessively contained, it adversely affects workability. Therefore, the upper limit is made 0.08% by mass.
Since S is a component harmful to corrosion resistance, it is necessary to reduce S as much as possible. Preferably it regulates to 0.005 mass% or less.

  Ni and Cu have an effect of improving the general corrosion resistance in an acidic atmosphere and improving the low temperature toughness of the ferritic stainless steel, and are added as necessary. However, since all of them are easily eluted, it is preferable to avoid adding a large amount. In particular, the eluted Ni poisons the catalyst and lowers the battery performance. Therefore, even when added, Ni is 0.5% by mass or less, preferably 0.15 to 0.35% by mass, and Cu is 0.8% by mass or less, preferably 0.20 to 0.50% by mass. % Range.

In addition, Ti and Nb which have the effect | action which fixes C and N in steel and improves workability may be added. In this case, both the Ti addition amount and the Nb addition amount are adjusted in the range of 0.03 to 0.25% by mass.
When Al is used for fixing N, the Al addition amount is adjusted in the range of 0.04 to 0.2% by mass.
V has the effect of improving the corrosion resistance in the internal environment of the fuel cell, and is added in the range of 0.2 to 0.1% by mass as necessary.

  When a high Cr-containing ferritic stainless steel sheet adjusted to a predetermined composition according to the above description is immersed in a non-oxidizing acid solution, a passive film having a higher Cr concentration than the base steel is formed on the steel sheet surface. . Non-oxidizing acid solutions such as hydrochloric acid and sulfuric acid are used as the dipping acid solution. Depending on the type of stainless steel, the acid type and concentration, or dipping conditions such as the liquid temperature and dipping time are selected. . For example, in the case of 30Cr steel, a condition of immersing in a hydrochloric acid bath of 10 to 20% by mass and a liquid temperature of 40 to 60 ° C. for 1 to 10 minutes is employed.

Depending on the type of stainless steel, the type and concentration of the acid solution, or the immersion conditions such as the liquid temperature and immersion time, the film thickness of the passive film to be formed and the Cr concentration in the passive film vary. The contact resistance with carbon paper also varies.
In order to obtain excellent power generation efficiency when incorporated as a separator of a fuel cell, contact resistance with carbon paper (for example, TGP-H-120 manufactured by Toray Industries, Inc.) that forms a gas diffusion layer to be an electrode, in the measured pressure 20kgf / cm ² 10mΩ · cm ² or less, preferably adjusted to below 5 m [Omega · cm ^2.
Therefore, depending on the type of stainless steel, the acid type and concentration, or the immersion conditions such as the liquid temperature and immersion time are selected so that the desired contact resistance is obtained.

Here, the measurement pressure of 20 kgf / cm ² is set as the contact resistance measurement condition for the following two reasons.
First, the deformation of the carbon paper constituting the electrode in contact with the separator was considered. When the fuel cell is fastened at a pressure of 20 kgf / cm ² , the carbon paper does not deform at all after 2000 hours of operation. However, when the fuel cell is fastened at a pressure of 40 kgf / cm ² , Indentation was observed.
Next, the deformation and breakage of the separator were considered. In the case of a stainless steel separator, there is no problem even if it is fastened at a pressure of 40 kgf / cm ² .
Therefore, it was judged that 20 kgf / cm ² is appropriate as the fastening pressure for preventing the deformation and breakage of the cell due to the long-term operation of the fuel cell when using the stainless steel separator.

Next, the reason why the contact resistance is specified to be 10 mΩ · cm ² or less will be described.
This is because if it is 10 mΩ · cm ² or less, the output loss due to the contact resistance is very small and hardly affects the output of the fuel cell. The reason why it is preferably set to 5 mΩ · cm ² or less is that it is considered to be a level that can counteract the contact resistance of a competing graphite separator or a stainless steel-plated separator.
Further, the reason why the contact resistance value after the wet test as the deterioration test is set to 25 mΩ · cm ² or less is that if the contact resistance is increased at this level, no significant output decrease is observed.

Using a total of six types of ferritic stainless steel plates, 42Cr steel, 38Cr steel, 30Cr steel, 22Cr steel, 18Cr steel, and 13Cr steel, the effect of acid solution immersion on contact resistance was investigated.
First, each of the stainless steel plates was immersed in a sodium orthosilicate solution having a concentration of 5% by mass and a liquid temperature of 60 ° C., and was subjected to cathodic electrolytic degreasing at a current density of 5 A / dm ² for 10 seconds, followed by washing with water and neutralization treatment. Next, the contact resistance was reduced by immersing in a hydrochloric acid solution having a concentration of 10% by mass and a liquid temperature of 50 ° C. for 3 minutes. Immediately after the immersion treatment, it was washed with water, desmutted, and dried with a dryer.

For the steel plate before the acid solution immersion treatment and the steel plate after the acid solution immersion treatment, a carbon paper (TGP-H-120, manufactured by Toray Industries, Inc.) was measured on a test piece cut out from each stainless steel plate, and the measurement pressure was 20 kgf / cm. contacting at ^2, to measure the contact resistance between the stainless steel plate / carbon paper.
As a result, acid contact resistance stainless steel the solution not subjected to immersion treatment, 38mΩ · cm ² in 42Cr steel, 50 m [Omega · cm ² in 38Cr steel, 70mΩ · cm ² in 30Cr steel, 120mΩ · cm ² in 22Cr steel, was 240mΩ · cm ² at 180mΩ · cm ^2, 13Cr steel 18Cr steel. In the stainless steel plate not subjected to the acid solution immersion treatment, it was confirmed that the contact resistance was lower as the Cr content was higher (see FIG. 1).
On the other hand, as shown in FIG. 1, the contact resistance was significantly reduced by the acid solution immersion treatment. 42Cr steel was 4mΩ · cm ² in 38Cr steel and 2 M [Omega · cm ² in 30Cr steel, 2.5mΩ · cm ² in 22Cr steel, 18Cr steel 3mΩ · cm ^2, 13Cr steel.

Next, each stainless steel plate after the acid solution immersion treatment was allowed to stand for 72 hours in a humid environment at a temperature of 70 ° C. and a relative humidity of 98%, assuming a wet environment in the fuel cell. The contact resistance between carbon papers was measured.
As a result, in the stainless steel plate having Cr of 22 mass% or more, no increase in contact resistance was observed after the wet test. On the other hand, the contact resistance slightly increased in the 18Cr stainless steel plate, and the contact resistance after the wet test increased significantly in the 13Cr stainless steel plate (see FIG. 1).

Assuming that the increase in contact resistance due to the wet test is due to the film thickness and alteration of the passive film, the 30DS steel and 13Cr steel are taken as an example, and the passive film on the stainless steel plate surface before and after the wet test is immersed in the GDS after immersion in the acid solution. Analysis was performed to determine the element distribution concentration in the depth direction.
The result is shown in FIG.
As can be seen from this result, in the 30Cr steel in which the increase in contact resistance was small, the film thickness of the passive film hardly changed before and after the wet test. On the other hand, in 13Cr steel with increased contact resistance, the passive film was thickened by the wet test.
Therefore, it has been found that the increase in contact resistance in a wet environment is caused by the thickening of the passive film.

Graph showing the relationship between the Cr content in stainless steel, the contact resistance of the untreated and acid-treated materials immersed in the acid solution, and the wet test after the acid solution immersion treatment Chart showing the results of GDS analysis of the passive film formed on the stainless steel plate surface before and after the wet test

Claims (4)

  Cr: 15 to 40 mass%, Mo: Ferritic stainless steel containing less than 1 mass% is used as a base material, and the entire surface is dipped in a non-oxidizing acid solution. Solid polymer fuel cell separator. The solid polymer fuel cell separator according to claim 1, wherein the contact resistance with carbon paper is 10 mΩ · cm ² or less at a measurement pressure of 20 kgf / cm ² . 3. The polymer electrolyte fuel cell according to claim 2, wherein the contact resistance with carbon paper after being left in 70 ° C. and 98% relative humidity for 72 hours is 25 mΩ · cm ² or less at a measurement pressure of 20 kgf / cm ² . Separator.   A polymer electrolyte fuel cell on which the separator according to claim 1 is mounted.

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