JP2005089800A - Stainless steel for polymer electrolyte fuel cell separator and polymer electrolyte fuel cell using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide stainless steel for a solid polymer electrolyte fuel cell separator, the steel having favorable corrosion resistance and low contact resistance (that is, excellent electric conductivity), and to provide a solid polymer electrolyte fuel cell which is obtained by using the stainless steel. <P>SOLUTION: The stainless steel has the composition comprising ≤0.03 mass% of C, 16 to 30 mass% of Cr, 7 to 40 mass% of Ni and the balance of Fe and inevitable impurities. The ratio of Cr/Fe calculated from the Cr content and the Fe content contained in a passive film on the surface of the stainless steel is ≥1 by the atomic ratio. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to stainless steel for a polymer electrolyte fuel cell separator having excellent durability and a small contact resistance value, and a polymer electrolyte fuel cell using the stainless steel separator.

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 reacts H ₂ and O ₂ to generate electricity, and its basic structure is an electrolyte membrane (ie, an ion exchange membrane), two electrodes (ie, a fuel electrode and an air electrode), and O ₂ ( That is, it comprises a diffusion layer of air) and H ₂ and two separators. Depending on the type of electrolyte membrane used, phosphoric acid fuel cells, molten carbonate fuel cells, solid electrolyte fuel cells, alkaline fuel cells, solid polymer fuel cells, and the like have been developed.

Among these fuel cells, solid polymer fuel cells are compared to phosphoric acid fuel cells and molten carbonate fuel cells.
(A) The power generation temperature is about 80 ° C, and it can generate power at a much lower temperature.
(B) The fuel cell body can be reduced in weight and size.
(C) Start up in a short time,
And so on. For this reason, the polymer electrolyte fuel cell is a fuel cell that is attracting the most attention today in order to be used as a power source for mounting an electric vehicle, a stationary generator for home use, and a small portable generator.

The polymer electrolyte fuel cell 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) and separators 4 and 4 are used. The membrane-electrode assembly 1 is sandwiched by 5 to form a single component (so-called single cell), and an electromotive force is generated between the separator 4 and the separator 5.

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

  When the polymer electrolyte fuel cell is applied to the above-described use, a fuel cell stack is formed by connecting several tens to several hundreds of such single cells in series.

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

  In terms of durability, when used as a power source for mounting on an electric vehicle, it is assumed to be about 5000 hours. Or, if it is used as a home-use generator, it is assumed that it is about 40,000 hours. Therefore, the separators 4 and 5 are required to have characteristics such as corrosion resistance that can withstand long-time power generation.

  In terms of electrical conductivity, it is desirable that the contact resistance between the separators 4 and 5 and the gas diffusion layers 2 and 3 is as low as possible. The reason is that as the contact resistance between the separators 4 and 5 and the gas diffusion layers 2 and 3 increases, the power generation efficiency of the polymer electrolyte fuel cell decreases. That is, the smaller the contact resistance, the better the electrical conductivity.

  To date, polymer electrolyte fuel cells using graphite as separators 4 and 5 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 easily damaged by an 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. These disadvantages of the separators 4 and 5 made of graphite are factors that 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 practical application of separators 4 and 5 made of stainless steel.

  For example, Japanese Patent Application Laid-Open No. 8-180883 discloses a technique that uses a metal that easily forms a passive film as a separator. However, the formation of a passive film causes 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 carbon material and inferior in corrosion resistance.

  Japanese Patent Application Laid-Open No. 10-228914 discloses a technique for reducing contact resistance and ensuring high output by performing gold plating on the surface of a metal separator such as SUS304. However, it is difficult to prevent pinholes with thin gold plating, and conversely with thick gold plating, the problem of cost remains.

  Japanese Patent Application Laid-Open No. 2000-277133 discloses a method of obtaining a separator having improved electrical conductivity by dispersing carbon powder in a ferritic stainless steel substrate. However, even when carbon powder is used, a cost problem still remains because the surface treatment of the separator requires a corresponding cost. Further, it has been pointed out that the separator subjected to the surface treatment has a problem that the corrosion resistance is remarkably lowered when scratches or the like are generated during assembly.

  Furthermore, an attempt has been made to apply stainless steel as it is to the separator without subjecting it to surface treatment. For example, in Japanese Patent Laid-Open Nos. 2000-239806 and 2000-294255, Cu, Ni are positively added, impurity elements such as S, P, and N are reduced, and C + N ≦ 0.03 mass%. , 10.5% by mass ≦ Cr + 3 × Mo ≦ 43% by mass is disclosed. In JP 2000-265248 A and JP 2000-294256 A, Cu and Ni are limited to 0.2% by mass or less to suppress elution of metal ions, and impurity elements such as S, P, and N are added. A ferritic stainless steel for a separator that is reduced and satisfies C + N ≦ 0.03% by mass, 10.5% by mass ≦ Cr + 3 × Mo ≦ 43% by mass is disclosed.

  However, all of these inventions define a stainless steel component within a predetermined range and strengthen the passive film, so that the surface treatment is not performed and the electrode-supported catalyst based on the eluted metal ions can be used as it is. This is based on the idea of reducing the deterioration of the catalytic ability of the steel and suppressing an increase in contact resistance with the electrode due to corrosion products. Therefore, it does not attempt to reduce the contact resistance of the stainless steel itself.

On the other hand, when the flow channel is formed by press molding, the austenitic stainless steel has better formability than the ferritic stainless steel. Therefore, an austenitic stainless steel having a low contact resistance value has been demanded as a material that can be used for press forming.
Japanese Patent Laid-Open No. 8-180883 Japanese Patent Laid-Open No. 10-228914 JP 2000-277133 A JP 2000-239806 JP JP 2000-294255 A JP 2000-265248 A JP 2000-294256 A

  In view of the above-mentioned problems of the prior art, the present invention is a stainless steel for a polymer electrolyte fuel cell separator having good corrosion resistance and low contact resistance (that is, excellent electrical conductivity). And a polymer electrolyte fuel cell using the same.

  That is, the present invention regulates not only the component of stainless steel as a raw material but also the component of the passive film existing on the surface within a predetermined range, so that the contact resistance is small and the power generation efficiency is excellent even without surface treatment. Another object of the present invention is to provide stainless steel for a polymer electrolyte fuel cell separator having high corrosion resistance of stainless steel itself, and a polymer electrolyte fuel cell using the same.

  The inventors of the present invention conducted intensive research on the austenitic stainless steel separator for exhibiting high corrosion resistance while keeping the contact resistance low, from the viewpoint of the components of the stainless steel and the surface passive film. As a result, it has been found that the contact resistance is greatly reduced by controlling the composition of the passive film formed on the surface of the austenitic stainless steel.

  First, the experimental results for conceiving the present invention will be described.

  In the experiment, a commercially available SUS316L (thickness 0.5 mm) was used as the raw material, and etching treatment was performed at various temperatures and times using an acidic aqueous solution containing 10% by mass nitric acid, 50% by mass hydrochloric acid, and 1% by mass picric acid. After that, it was washed with pure water and dried with cold air, and subjected to measurement of contact resistance.

For the measurement of contact resistance, four test pieces (50 mm × 50 mm) etched under the same conditions were prepared, and two test pieces 8 as shown in FIG. Carbon paper (Toray TGP-H-120) 9 is alternately sandwiched, and the copper plate is contacted with gold-plated electrodes 10, and a pressure of 137.2 N / cm ² (ie, 14 kgf / cm ² ) is applied per unit area. The resistance between the test pieces 8 was measured. The value obtained by multiplying the measured value by the area of the contact surface and further dividing by the number of contact surfaces (= 2) was taken as the contact resistance value.

  The contact resistance values were calculated based on the measurement values measured six times while exchanging a set of two test pieces 8, and the average values are shown in Table 1.

  As a reference example, the same measurement was performed on a stainless steel plate (thickness 0.3 mm, equivalent to SUS304) and a graphite plate (thickness 5 mm) with gold plating (thickness approx. 0.1 μm) on the surface, and the contact resistance value was calculated. Calculated. The results are also shown in Table 1.

  Further, the Cr content and the Fe content of the passive film after the etching treatment were measured, and the Cr / Fe ratio was calculated. The results are shown in Table 1. The Cr content and the Fe content were obtained from the spectral intensities of Cr and Fe using Auger electron spectroscopy, and the atomic ratio was calculated as the Cr / Fe ratio.

As is apparent from Table 1, the contact resistance value is lowered by etching the stainless steel plate. In particular, if the Cr / Fe ratio of the passive film is 1 or more in terms of the number of atoms, the contact resistance value of the stainless steel plate is 10 mΩ · cm ² or less, which is equivalent to that of the gold-plated stainless steel plate or graphite plate. A value is obtained. If the contact resistance value is 10 mΩ · cm ² or less, the fuel cell characteristics are hardly adversely affected.

  Until now, there has been an example in which the influence of the Cr / Fe ratio of the passive film has been investigated from the viewpoint of corrosion resistance, but this experiment also showed that the contact resistance value of austenitic stainless steel can be greatly increased by adjusting the components of the passive film. We have obtained an unprecedented knowledge that it can be reduced. The present invention completed based on this finding is as follows.

  That is, the present invention is an austenitic stainless steel having a composition containing 0.03% by mass or less of C, 16-30% by mass of Cr, 7-40% by mass of Ni, and the balance consisting of Fe and inevitable impurities, In addition, this is a stainless steel for polymer electrolyte fuel cell separators in which the Cr / Fe ratio calculated from the Cr content and the Fe content contained in the passive film on the surface of the stainless steel is 1 or more in terms of the atomic ratio.

  In the present invention, the stainless steel preferably contains one or more selected from the following groups (1) to (3) in addition to the composition described above.

(1) Mo: 10 mass% or less
(2) N: 0.4% by mass or less
(3) Cu: 3% by mass or less In addition to the above-described composition, it is preferable to contain one or more selected from the following groups (a) to (b).

(a) Si: 1.5% by mass or less
(b) Mn: 2.5% by mass or less The present invention is a solid polymer fuel cell comprising a solid polymer membrane, an electrode and a separator, and the above stainless steel for a polymer electrolyte fuel cell separator is used as the separator. This is a polymer electrolyte fuel cell.

  According to the present invention, a stainless steel for a polymer electrolyte fuel cell separator having a low contact resistance value and excellent corrosion resistance can be obtained. Therefore, it has become possible to provide an inexpensive stainless steel separator for a polymer electrolyte fuel cell that has conventionally used an expensive graphite separator due to durability problems.

  The present invention is not limited to the polymer electrolyte fuel cell separator, and can be widely used as an electrical component made of stainless steel having electrical conductivity.

  First, the reasons for limiting the components of the separator stainless steel according to the present invention will be described.

C: 0.03 mass% or less C reacts with Cr in stainless steel and precipitates as Cr carbides at grain boundaries, resulting in a decrease in corrosion resistance. Therefore, the smaller the content of C, the better. If C is 0.03% by mass or less, the corrosion resistance is not significantly reduced. In addition, Preferably it is 0.015 mass% or less.

Cr: 16-30% by mass
Cr is an element necessary for ensuring basic corrosion resistance as a stainless steel plate. If the Cr content is less than 16% by mass, it cannot be used for a long time as a separator. On the other hand, when the Cr content is less than 16% by mass, it is difficult to adjust the Cr / Fe ratio of the passive film to 1 or more and to make the contact resistance value 10 mΩ · cm ² or less. On the other hand, if the Cr content exceeds 30% by mass, it is difficult to generate an austenite phase. Therefore, the Cr content is 16-30% by mass. In addition, Preferably it is 17-26 mass%.

Ni: 7-40% by mass
Ni is an element that stabilizes the austenite phase and is added to generate the austenite phase. If the Ni content is less than 7% by mass, the effect of stabilizing the austenite phase cannot be obtained. On the other hand, when the Ni content exceeds 40% by mass, excessive consumption of Ni causes an increase in cost. Therefore, the Ni content is 7 to 40% by mass.

  In the present invention, Mo or N can be appropriately added as described later. Mo and Cr are elements that stabilize the ferrite phase. N, like Ni, is an element that stabilizes the austenite phase. Therefore, it is preferable to adjust the Ni content within the range of 7 to 40% by mass in accordance with the amount of Cr, Mo and N added.

  In the stainless steel for a separator of the present invention, the following elements may be added as needed in addition to C, Cr, and Ni.

One or more kinds selected from the group of Mo: 10 mass% or less, N: 0.4 mass% or less, and Cu: 3 mass% or less
Mo: 10 mass% or less
Mo is an element effective for suppressing local corrosion such as crevice corrosion of a stainless steel plate. However, if added in excess of 10% by mass, the stainless steel becomes significantly brittle and productivity is lowered, and excessive consumption of Mo causes an increase in cost. Therefore, when adding Mo, 10 mass% or less is preferable. However, 1-8 mass% is still more preferable.

N: 0.4 mass% or less N is also an element effective for suppressing local corrosion such as crevice corrosion of stainless steel sheets. However, if added over 0.4% by mass, it takes a long time to add N in the melting stage of stainless steel, so that productivity is lowered and formability of the steel sheet is lowered. Therefore, when adding N, 0.4 mass% or less is preferable. However, 0.01-0.3 mass% is still more preferable.

Cu: 3% by mass or less
Cu is an element effective for improving the corrosion resistance of a stainless steel plate. However, when it is added in excess of 3% by mass, not only the hot workability is deteriorated and the productivity is lowered, but also excessive addition of Cu causes an increase in cost. Therefore, when adding Cu, 3 mass% or less is preferable. However, 0.01-2.5 mass% is still more preferable.

One or more kinds selected from the group of Si: 1.5% by mass or less and Mn: 2.5% by mass or less
Si: 1.5% by mass or less
Si is an effective element for deoxidation, and is added at the melting stage of stainless steel. However, if excessively contained, the stainless steel sheet becomes hard and ductility is lowered. Therefore, when Si is added, the content is preferably 1.5% by mass or less. However, 0.01-1.0 mass% is still more preferable.

Mn: 2.5% by mass or less
Mn combines with the unavoidably mixed S and has the effect of reducing S dissolved in the stainless steel, so it is effective for suppressing grain boundary segregation of S and preventing cracking during hot rolling. It is an element. Such an effect is exhibited when the content is 2.5% by mass or less. Therefore, when adding Mn, 2.5 mass% or less is preferable. However, 0.01-2.0 mass% is still more preferable.

  In the present invention, in addition to the elements described above, 1.0% by mass or less of Ti and Nb may be added to improve intergranular corrosion resistance. Moreover, 0.2% by mass or less of Al may be added as a deoxidizing element.

  Other elements are the balance Fe and inevitable impurities.

  Next, the characteristics that the stainless steel for separators according to the present invention should have will be described.

The Cr / Fe ratio of the passive film on the stainless steel plate surface is 1 or more in terms of atomic ratio. The components of the passive film are an important factor in reducing the contact resistance value. It is necessary to increase the Cr / Fe ratio calculated from the Cr content and the Fe content of the active film. As explained in the above experimental results, in order to obtain a contact resistance value of 10 mΩ · cm ² or less, the Cr / Fe ratio needs to be 1 or more in terms of the number of atoms.

  Thus, in order to adjust the Cr content and the Fe content of the passive film, techniques such as etching using an acid, immersion in an acidic aqueous solution, and electrolytic etching can be used.

  Adjustment of the Cr content and the Fe content of the passive film may be performed before the stainless steel plate is processed into a separator, or may be performed after the stainless steel plate is processed into a separator. However, in order to stably maintain the Cr / Fe ratio at 1 or more, it is preferable to perform pickling after the separator is processed.

  When a polymer electrolyte fuel cell is produced using the stainless steel separator thus produced, a polymer electrolyte fuel cell having a low contact resistance value, excellent power generation efficiency, and high corrosion resistance can be produced.

  Stainless steels having the components shown in Table 2 were melted by a converter and secondary refining, and further slabs having a thickness of 200 mm were formed by a continuous casting method. After heating this slab to 1250 degreeC, it was made into the hot rolled stainless steel plate of thickness 4mm by hot rolling, and also annealed (950-1200 degreeC) and the pickling process. Subsequently, the thickness was reduced to 0.3 mm by cold rolling, and after annealing (950 to 1150 ° C.) and pickling treatment, temper rolling was performed to obtain a so-called 2B-finished cold rolled stainless steel plate (thickness 0.2 mm).

  Four test pieces each having a size of 200 mm × 200 mm were cut out from the central portion of the obtained cold-rolled stainless steel plate and the central portion in the longitudinal direction. Four test pieces cut out from each of the cold-rolled stainless steel plates of steel numbers 1 to 8 were subjected to press working to obtain a separator having a predetermined shape. Thereafter, a passive film adjustment treatment was applied to some separators for each steel number to adjust the Cr / Fe ratio. Here, when performing the passive film adjustment treatment, A: 10% by mass of nitric acid, 50% by mass of hydrochloric acid and 1% by mass of picric acid (60 ° C., 120 seconds) or B: 10% by mass of nitric acid And a solution containing 60% by mass of hydrofluoric acid (60 ° C., 300 seconds) was used.

  When the passive film adjustment treatment is not performed after the press working, the Cr content and the Fe content of the passive film after the press work are measured, and the treatment liquid A is applied after the press work. When the passive film adjustment treatment is used and when the passive film adjustment treatment is performed using the treatment liquid B after press working, the Cr content of the passive film after the passive film adjustment treatment The Fe content was measured and the Cr / Fe ratio was calculated for each. The Cr content and the Fe content were obtained from the spectral intensities of Cr and Fe using Auger electron spectroscopy, and the atomic ratio was calculated as the Cr / Fe ratio.

Thus, the power generation characteristic was investigated using the separator which performed the passive film adjustment process, and the separator which did not process, respectively. For evaluation of power generation characteristics, a membrane-electrode assembly 1 (FC50-MEA manufactured by Electrochem Co., Ltd.) having an effective area of 50 cm ² in which a polymer membrane and an electrode and gas diffusion layers 2 and 3 are integrated is used. A single cell having the shape shown in FIG. 1 was prepared. The single cell air flow path 6 and hydrogen flow path 7 were both rectangular with a height of 1 mm and a width of 2 mm, and were arranged in 17 rows in total. Air is supplied to the cathode side, and ultra high purity hydrogen (purity 99.9999 vol%) is supplied to the anode side after being humidified by a bubbler maintained at 80 ± 1 ° C, and an output voltage with a current density of 0.4 A / cm ² is supplied. It was measured.

  Moreover, after operating continuously for 2000 hours on the same conditions, the output voltage was measured. During the period of the power generation experiment of this single cell, the temperature of the single cell main body was maintained at 80 ± 1 ° C. The membrane-electrode assembly 1, carbon paper 9 and the like were replaced with new ones every time the test piece was changed.

As a reference example, a stainless steel plate (equivalent to SUS304) is processed into the same shape as steel numbers 1 to 8 above, and then a gold plated (thickness: about 0.1 μm) surface is applied to the separator, and a 3 mm thick graphite plate An output voltage with a current density of 0.4 A / cm ² was measured using a separator in which 17 rows of grooves having a width of 2 mm and a height of 1 mm were arranged on one side of the plate at intervals of 2 mm. The method for measuring the output voltage is the same as the steel numbers 1 to 8 described above.

  The results are shown in Table 3.

  As is apparent from Table 3, stainless steel (that is, steel numbers 1 to 6) satisfying the component range of the present invention is treated with the liquid A or B, and the components of the passive film are adjusted to obtain Cr / Fe. A single cell using a separator with a ratio of 1 or more obtained an output voltage equivalent to that of a gold-plated SUS304 separator or a graphite plate separator for both the initial output voltage and the output voltage after 2000 hours. .

  On the other hand, in the case of stainless steel (ie, steel numbers 7 to 9) outside the component range of the present invention, the initial output voltage and the output voltage after 2000 hours are both gold-plated regardless of the presence or absence of the passive film adjustment treatment. Compared with SUS304 separator and graphite plate separator.

  Further, even when the stainless steel (that is, steel numbers 1 to 6) satisfying the component range of the present invention is not subjected to the passive film adjustment treatment, the passive film has a low Cr / Fe ratio and an initial output voltage. Decreased compared to the SUS304 separator and the graphite plate separator plated with gold.

It is a perspective view which shows typically the example of the single cell of a polymer electrolyte fuel cell. It is sectional drawing which shows a continuous casting machine typically.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Membrane-electrode assembly 2 Gas diffusion layer 3 Gas diffusion layer 4 Separator 5 Separator 6 Air flow path 7 Hydrogen flow path 8 Test piece 9 Carbon paper
10 electrodes

Claims (4)

  A stainless steel containing 0.03 mass% or less of C, 16-30 mass% of Cr, 7-40 mass% of Ni, and the balance consisting of Fe and inevitable impurities, and the surface of the stainless steel A stainless steel for a polymer electrolyte fuel cell separator, characterized in that the Cr / Fe ratio calculated from the Cr content and the Fe content contained in the passive film is 1 or more in terms of atomic ratio. 2. The stainless steel for polymer electrolyte fuel cell separators according to claim 1, wherein the stainless steel contains at least one selected from the group consisting of the following (1) to (3) in addition to the composition: steel.
(1) Mo: 10 mass% or less
(2) N: 0.4% by mass or less
(3) Cu: 3 mass% or less 3. The polymer electrolyte fuel cell separator according to claim 1, wherein the stainless steel contains at least one selected from the following groups (a) to (b) in addition to the composition: For stainless steel.
(a) Si: 1.5% by mass or less
(b) Mn: 2.5% by mass or less   A polymer electrolyte fuel cell comprising a polymer electrolyte membrane, an electrode, and a separator, wherein the separator uses the stainless steel for a polymer electrolyte fuel cell separator according to any one of claims 1, 2, and 3. A solid polymer fuel cell.

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