JP5375191B2 - Stainless steel and polymer electrolyte fuel cell for polymer electrolyte fuel cell separator with low ion elution - Google Patents

Description

  The present invention relates to a stainless steel for a fuel cell separator with a small amount of ion elution at a high potential, and a polymer electrolyte fuel cell using such stainless steel as a separator for a fuel cell.

  In recent years, fuel cells that are excellent in power generation efficiency and do not emit carbon dioxide have been developed from the viewpoint of global environmental conservation. This fuel cell reacts hydrogen and oxygen to generate electricity, and its basic structure has a sandwich-like structure, an electrolyte membrane (ion exchange membrane), two electrodes (a fuel electrode and Air electrode), a diffusion layer of hydrogen and oxygen (air), and two separators. Depending on the type of electrolyte used, it is classified into phosphoric acid form, molten carbonate form, solid oxide form, alkali form, and solid polymer form.

Among the fuel cells described above, the polymer electrolyte fuel cell is compared with the molten carbonate fuel cell or the phosphoric acid fuel cell.
(1) The operating temperature is extremely low, around 80 ℃,
(2) The battery body can be reduced in weight and size.
(3) It has features such as quick start-up, high fuel efficiency and high power density.
For this reason, the polymer electrolyte fuel cell is one of the fuel cells that are attracting the most attention today as a power source for mounting an electric vehicle and a small distributed power source (stationary small generator) for home use and business use. is there.

  The polymer electrolyte fuel cell is based on the principle of taking out electricity from hydrogen and oxygen through a polymer membrane. As shown in FIG. 1, the structure of the polymer polymer fuel cell is platinum-based on the front and back surfaces of the membrane. Membrane-Electrode Assembly (MEA: Membrane-Electrode Assembly, thickness: several tens to several hundreds μm) 1 integrated with electrode material such as carbon black carrying a catalyst, gas diffusion layers 2 and 3 such as carbon cloth, and separator 4 and 5 is used as a single component (single cell), and an electromotive force is generated between the separators 4 and 5. At this time, the gas diffusion layer is often integrated with the MEA. A fuel cell stack is configured and used by connecting several tens to several hundreds of such single cells in series.

In addition to the role as a partition that separates single cells,
(a) a conductor carrying the generated electrons,
(b) Oxygen (air) and hydrogen flow paths (air flow path 6 and hydrogen flow path 7 in FIG. 1 respectively) and generated water and exhaust gas discharge paths (air flow path 6 and hydrogen flow in FIG. 1 respectively) Channel 7)
Function is required.
As for durability, about 5,000 hours are assumed for an automobile fuel cell, and about 40,000 hours are assumed for a stationary fuel cell used as a small-sized distributed power source for home use.

  Solid polymer fuel cells that have been put to practical use up to now have been provided using carbon materials as separators. However, this carbon separator is not only easily damaged by an impact, but also has a drawback that it is difficult to make it compact and the processing cost for forming the flow path is high. In particular, the cost problem is the biggest obstacle to the spread of fuel cells.

Therefore, an attempt has been made to apply a metal material, particularly stainless steel, instead of the carbon material.
When stainless steel is applied, corrosion resistance and electrical conductivity (low contact resistance) are required.
Regarding corrosion resistance, for example, Patent Document 1 discloses a stainless steel for a separator in which the content of Cr and Mo is increased to improve the corrosion resistance from the component side.
As for conductivity, methods for coating various conductive substances on the surface of stainless steel have been proposed. In this regard, the inventors have also proposed a technique using a Laves phase as a conductive precipitate in Patent Document 2.
In addition, for example, Patent Document 3 discloses a technique of using a metal that easily forms a passive film as a separator, and Patent Document 4 discloses a method in which carbon powder is dispersed and adhered to a ferritic stainless steel base material to provide conductivity ( technique to obtain a separator having improved contact resistance) further Patent Document 5, reduced separator contact resistance the M ₂₃ C ₆ type carbide or M ₂ B-type boride is precipitated on the surface is disclosed.

  However, as shown in Non-Patent Document 1, for example, a potential exceeding 1 V is locally and temporarily generated on the cathode (air electrode) side due to non-uniformity of the reaction in recent years. It has been pointed out that there may be cases. When the potential exceeds 1 V, it becomes a hyperpassivation region of stainless steel, so that elution of metal ions from stainless steel increases and there is a risk of causing performance degradation of the electrolyte membrane and electrode catalyst. With respect to this problem, it is difficult to suppress ion elution from stainless steel at a high potential with the above-described conventional technology.

  In this regard, Patent Document 6 discloses a technique for ensuring high output by reducing contact resistance by performing gold plating on the surface of a metal separator such as SUS304. In this case, although ion elution at a high potential can be reduced, it is difficult to prevent the occurrence of pinholes in thin gold plating, and conversely, a problem of cost remains in thick gold plating.

JP 2000-239806 JP JP 2007-254794 A Japanese Patent Laid-Open No. 8-180883 JP 2000-277133 A Japanese Unexamined Patent Publication No. 2000-214186 Japanese Patent Laid-Open No. 10-228914

C.A.Reiser et al., Electrochem.Solid-State Lett., Vol.8, p.A273-A276 (2005)

  The present invention has been developed in view of the above-mentioned problems of the prior art. A solid polymer fuel cell using a stainless steel for a fuel cell separator with a small ion elution amount in a high potential region. It aims to be proposed together.

Now, in order to solve the above-mentioned problems, the inventors have conducted a detailed study from the component aspect on a method for reducing the ion elution amount (corrosion current) in a high potential region. as a result,
(1) Increasing the Cr content increases the corrosion current in the high potential range.
(2) Increasing the Mo content further increases the corrosion current in the high potential region.
(3) In contrast, it has been found that increasing the Nb content can suppress the increase in the corrosion current described above.
Furthermore, by coating the stainless steel surface with a conductive material or exposing conductive precipitates, the contact resistance is reduced, but when the corrosion current in the high potential region is reduced, the surface is formed by corrosion. It has also been found that since the film becomes thinner, the effect of reducing the contact resistance by the conductive substance can be maintained for a longer period of time.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.03% or less,
N: 0.03% or less,
Si: 0.01-2.0%
Mn: 0.01 to 2.0%
Al: 0.001 to 0.3%,
Cr: 20-35%
Mo: 4.0% or less and
Nb: 0.51 to 2.0%
The balance consists of Fe and inevitable impurities, and the Cr amount, Nb amount and Mo amount satisfy the relationship of the following formulas (1) and (2), and in an aqueous solution of sulfuric acid at pH 3 at 80 ° C. The maximum current density at the 20th time when the range of 0.8V vs. SHE and 1.2V vs. SHE is repeatedly polarized at a sweep rate of 20mV / min is 1μA / cm ² or less . Stainless steel for polymer electrolyte fuel cell separator with low ion elution.
Record
Cr + 3Mo ≧ 23 --- (1)
Nb / (Cr + 5Mo) ≧ 0.01 --- (2)

2. In addition to the above ingredients,
1 or 2 types selected from Ti and Zr: 1.0% or less, The stainless steel as described in 1 above.

3 . 3. The stainless steel as described in 1 or 2 above, wherein a conductive substance is present on the surface.

4 . 4. The stainless steel as described in 3 above, wherein the conductive material is an Fe ₂ Nb type Laves phase.

5 . A solid polymer fuel cell comprising a solid polymer membrane, an electrode, a gas diffusion layer and a separator, wherein the stainless steel according to any one of the above 1 to 4 is used as the separator. Fuel cell.

According to the present invention, it is possible to obtain a stainless steel for a fuel cell separator with a small amount of ion elution even when the cathode electrode side is at a high potential without using a graphite plate or a gold-plated stainless steel plate.
As a result, it is possible to provide an inexpensive stainless steel separator for a fuel cell that has conventionally had to use an expensive carbon separator or a gold plating separator, and a significant reduction in manufacturing cost can be achieved.

It is a schematic diagram which shows the basic structure of a fuel cell. It is a graph which shows the relationship between Nb / (Cr + 5Mo) value and current density (μA / cm ² ).

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the stainless steel for a separator according to the present invention is limited to the above range will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.03% or less, N: 0.03% or less Both C and N form a compound with Cr in the steel, and precipitate as Cr carbonitride at the grain boundary, resulting in a decrease in corrosion resistance. For this reason, it is preferable that both elements are low, but if C is 0.03% or less and N is 0.03% or less, the corrosion resistance is not significantly reduced. For this reason, it was limited to C: 0.03% or less and N: 0.03% or less. Preferably, C is 0.015% or less, and N is 0.015% or less.

Si: 0.01-2.0%
Since Si is an effective element for deoxidation, it is added for this purpose. However, to obtain this effect, addition of at least 0.01% is necessary. On the other hand, if excessively contained, the steel sheet is hardened and the ductility is lowered, so the upper limit of the Si content is set to 2.0%. Preferably it is 0.05 to 1.0%.

Mn: 0.01-2.0%
Mn is an effective element that combines with S and reduces the solid solution S to suppress grain boundary segregation of S and prevent cracking during hot rolling. In order to obtain this effect, addition of at least 0.01% is necessary, but since the effect is saturated even if it exceeds 2.0%, the amount of Mn is limited to a range of 0.01 to 2.0%. Preferably it is 0.05 to 1.0% of range.

Al: 0.001 to 0.3%
Al contributes to deoxidation in the steelmaking process, but if the content is less than 0.001%, the effect of addition is poor, while if it exceeds 0.3%, the effect reaches saturation, and rather costs increase, so Al The amount was limited to a range of 0.001 to 0.3%. Preferably it is 0.01 to 0.2% of range.

Cr: 20-35%
Cr is an element necessary for ensuring basic corrosion resistance as stainless steel. The operating environment of the separator varies depending on the structure and operation method of the fuel cell. However, considering fluctuations in the operating environment during actual use, if the Cr content is less than 20%, it cannot withstand long-term use as a separator, while the Cr content is 35 If it exceeds 50%, there is a disadvantage that the productivity is significantly reduced. Therefore, the Cr content is limited to a range of 20 to 35%. Preferably it is 24 to 35% of range.

Mo: 4.0% or less
Mo is an element effective for improving the corrosion resistance, especially the local corrosion resistance of stainless steel. Therefore, it is preferable to contain 0.1% or more, but if the amount of Mo exceeds 4.0%, not only the productivity is lowered but also the cost is increased. Therefore, the Mo content is 4.0% or less. Preferably it is 2.0% or less.

Nb: 0.51 to 2.0%
Nb is an element effective for fixing C and N in steel as carbonitrides and improving corrosion resistance and press formability. Further, by adding 0.51 % or more of Nb, a conductive Laves phase can be precipitated on the surface of the stainless steel to improve electrical conductivity. On the other hand, if it exceeds 2.0%, the manufacturability and moldability are lowered. For this reason, the amount of Nb was limited to the range of 0.51 to 2.0%. Preferably it is 0.51 to 1.0% of range.

As described above, the proper composition range of the basic component of the present invention has been described. However, in the present invention, it is not sufficient that each element simply satisfies the above range, and Cr, Mo, and Nb satisfy the following relationship. It is important to let
Cr + 3Mo: 23% or more
As described above, Mo is an element effective for improving the corrosion resistance of stainless steel, particularly the local corrosion resistance. The operating environment of the separator varies depending on the structure and operation method of the fuel cell, but considering the fluctuation of the operating environment at the time of actual use, it is necessary to contain it within a range satisfying Cr + 3Mo ≧ 23. If (Cr + 3Mo) is less than 23%, the corrosion resistance is lowered when the pH is lowered during operation.

Nb / (Cr + 5Mo): 0.01% or more
An increase in Cr and Mo increases the corrosion current in the high potential region that occurs in the unsteady operation mode, so in order to suppress this, the combined addition of Nb is indispensable. Here, since the increase in the corrosion current in the high potential region is more affected by Mo than Cr, it is necessary to contain Nb so as to satisfy Nb / (Cr + 5Mo) ≧ 0.01.

FIG. 2 shows the results of examining the relationship between the Nb / (Cr + 5Mo) value and the current density (μA / cm ² ). This current density is the maximum current density of the 20th time when the range of 0.8V vs. SHE and 1.2V vs. SHE is repeatedly polarized at a sweep rate of 20mV / min in an aqueous solution of sulfuric acid at pH 3 at 80 ° C. That is, the change of the operating environment at the time of actual use is taken into consideration.
As is clear from the figure, by setting the Nb / (Cr + 5Mo) value to 0.01 or more, even if the operating environment fluctuates during actual use, the potential increase can be suppressed to 1 V or less. The ion elution amount (corrosion current) in the high potential region, which has been a concern in the past, can be effectively reduced.

The basic components of the present invention have been described above. However, in the present invention, the following elements can be appropriately contained as necessary.
One or two selected from Ti and Zr: 1.0% or less
Ti and Zr are both effective elements for fixing C and N in steel as carbonitrides and improving corrosion resistance and press formability. However, if Ti and Zr are added in excess of 1.0% in total, the steel sheet becomes significantly embrittled and productivity decreases. Therefore, Ti and Zr should be contained at 1.0% or less in either case of single addition or composite addition.

  In the present invention, in addition to the above elements, one or more selected from Ca, Mg, REM and B can be added at 0.1% or less for the purpose of improving hot workability. . Further, for the purpose of improving the corrosion resistance, Ni: 1.0% or less and Cu: 5.0% or less can be added. Other elements are the balance Fe and inevitable impurities.

There is no restriction | limiting in particular about the manufacturing method of above-mentioned stainless steel, What is necessary is just to follow a conventionally well-known method. The preferred production conditions are as follows.
The steel slab adjusted to the above preferred component composition is heated to 1150 ° C. or higher, then hot rolled, then annealed at a temperature of 1000 to 1100 ° C., further cold rolled, and then 950 to 1100 ° C. Bright annealing at temperature.
Thus, a stainless steel plate having a thickness of about 0.05 to 0.3 mm is obtained.

Thus, a stainless steel for a polymer electrolyte fuel cell separator with a small ion elution amount can be obtained.
Here, “low ion elution amount” means that the range of 0.8 V vs. SHE and 1.2 V vs. SHE is repeatedly polarized at a sweep rate of 20 mV / min in an aqueous solution of sulfuric acid at pH 3 at 80 ° C. This means that the 20th maximum current density is 1 μA / cm ² or less.

Furthermore, in this invention, in order to provide electroconductivity, it is preferable to coat an electroconductive substance on the surface or to expose electroconductive deposits on the surface. As the coating, gold, graphite, nitrides such as TiN, carbides such as TiC, and the like are advantageously suitable. In addition, since the steel of this invention has the outstanding corrosion resistance, this coating does not need to cover the whole surface of a steel plate.
Moreover, when using a conductive precipitate, since the steel of the present invention contains Nb, a Fe ₂ Nb type Laves phase can be used. The Laves phase can be precipitated by an aging treatment at 700 to 950 ° C., but can also be precipitated by controlling the cooling rate after heat treatment or heat treatment in the steel plate production process. Further, the aging treatment may be performed after processing into a separator. If the surface is etched by pickling after the precipitation treatment, a surface with low contact resistance can be obtained.

  By applying the stainless steel obtained as described above to the separator of the polymer electrolyte fuel cell, a polymer electrolyte fuel cell with a small amount of ion elution at a high potential can be obtained.

Steel with the chemical composition shown in Table 1 is melted by vacuum melting, and the resulting ingot is heated to 1150 ° C or higher, then hot rolled into a hot-rolled sheet with a thickness of 5 mm and heated at 1000 to 1100 ° C. The steel sheet was annealed and pickled. Next, cold rolling, annealing and pickling were repeated to obtain a cold-rolled annealed sheet having a thickness of 0.3 mm. In addition, some steel plates (No. 1, 7, 13, 19, and 24) were subjected to aging treatment at 800 ° C for 10 minutes to reduce the contact resistance, and then Fe ₂ Nb type Laves phase was precipitated. The laves phase was exposed to the steel sheet surface by pickling with nitric hydrofluoric acid.

A 30 mm square test piece was cut out from a cold-rolled annealed sheet and an aging-treated steel sheet, degreased with acetone, and simulated in a pH 3 sulfuric acid (80 ° C) simulating the operating environment of a fuel cell. The range of 0.8V vs. SHE and 1.2V vs. SHE was repeatedly polarized at a sweep rate of 20mV / min. At this time, the current density at each potential was measured. Although the current density decreased due to repetitive polarization, a sample having a maximum current of 1 μA / cm ² or less at the 20th potential sweep was judged to have good corrosion resistance at a high potential.
The obtained results are also shown in Table 1.

For the aging-treated steel sheet, the sample after 30 times of polarization treatment was washed with pure water, blown and dried, and then contact resistance was measured. The contact resistance is measured by sandwiching both sides of the test piece after polarization measurement with 10 mm x 10 mm carbon paper (Toray: TGP-H-120), and then contacting a gold-plated electrode on a copper plate of the same area. Per unit area: It was measured by applying a pressure of 196 MPa (20 kgf / cm ² ) and measuring the resistance between the gold plating electrode on the polarization measuring surface side and the test piece. If the contact resistance value is 10 mΩ · cm ² or less, the conductivity is good.
The obtained results are shown in Table 2.

As shown in Table 1, all of the inventive examples (Nos. 4-5, 7-8, 11, 13-14, 16-17 ) have a maximum current density of 1 μA / cm ² or less during 20 times polarization measurement. It can be seen that the corrosion resistance at high potential is excellent. On the other hand, in all of the comparative examples (Nos. 18 to 27) in which the Nb content relative to the Cr amount and the Mo amount is less than the appropriate amount, the maximum current density at the time of repeated 20 times is over 1 μA / cm ² . Good high-potential corrosion resistance was not obtained.
Further, as is clear from Table 2, it can be seen that all of the inventive examples (Nos. 7 and 13) have a contact resistance value of 10 mΩ · cm ² or less, and good conductivity is obtained. The reason for this is that the steel according to the present invention has a low ion elution amount at a high potential, that is, a small corrosion current, resulting in a decrease in corrosion products generated on the surface, resulting in a decrease in contact resistance after repeated polarization. It is done.
On the other hand, all of the comparative examples (Nos. 19 and 24) had a contact resistance value exceeding 10 mΩ · cm ² .

Claims (5)

% By mass
C: 0.03% or less,
N: 0.03% or less,
Si: 0.01-2.0%
Mn: 0.01 to 2.0%
Al: 0.001 to 0.3%,
Cr: 20-35%
Mo: 4.0% or less and
Nb: 0.51 to 2.0%
The balance consists of Fe and inevitable impurities, and the Cr amount, Nb amount and Mo amount satisfy the relationship of the following formulas (1) and (2), and in an aqueous solution of sulfuric acid at pH 3 at 80 ° C. The maximum current density at the 20th time when the range of 0.8V vs. SHE and 1.2V vs. SHE is repeatedly polarized at a sweep rate of 20mV / min is 1μA / cm ² or less . Stainless steel for polymer electrolyte fuel cell separator with low ion elution.
Record
Cr + 3Mo ≧ 23 --- (1)
Nb / (Cr + 5Mo) ≧ 0.01 --- (2) In addition to the above ingredients,
The stainless steel according to claim 1, containing 1.0% or less of one or two selected from Ti and Zr. Stainless steel according to claim 1 or 2, wherein a conductive material is present on the surface. The stainless steel according to claim 3 , wherein the conductive material is an Fe ₂ Nb type Laves phase. A solid polymer fuel cell comprising a solid polymer membrane, an electrode, a gas diffusion layer, and a separator, wherein the stainless steel according to any one of claims 1 to 4 is used as the separator. Fuel cell.

Images