JP4322136B2 - Ferritic stainless steel for polymer electrolyte fuel cell separator - Google Patents

Description

  The present invention relates to a ferritic stainless steel suitable for a separator incorporated in a polymer electrolyte fuel cell that is promising as an in-vehicle power source, a home cogeneration system, and the like.

Fuel cells include phosphoric acid type, molten carbonate type, solid polymer type, solid electrolyte type, etc., but there is little emission of CO ₂ , NOx, SOx, etc., and solid polymer showing very high power generation efficiency Type fuel cells are promising. Solid polymer fuel cells are capable of operating at a low temperature of 100 ° C. or less and can be started up in a short time, and therefore are beginning to be adopted as power sources for automobiles, stationary devices, mobile devices and the like.
The fuel cell is assembled by stacking several tens to several hundreds of unit cells of the minimum unit in order to extract electric power for practical use. Each unit cell utilizes the fact that an ion exchange membrane made of a solid polymer resin having a proton exchange group functions as a proton-conductive electrolyte, fuel gas on one side of the ion exchange membrane, air on the other side, It has a structure for flowing an oxidizing gas such as oxygen.

Specifically, an oxidation electrode 2 and a fuel electrode 3 are joined to both sides of the ion exchange membrane 1, and a separator 5 is opposed to each of the oxidation electrode 2 and the fuel electrode 3 via a gasket 4 (FIG. 1a). An oxidizing gas supply port 6 and a discharge port 7 are formed in the separator 5 on the oxidation electrode 2 side, and a fuel gas supply port 8 and a discharge port 9 are formed in the separator 5 on the fuel electrode 3 side. In addition, a plurality of grooves 5g are formed in the separator 5 to achieve conduction and uniform distribution of the fuel gas g and the oxidizing gas o (FIG. 1b).
In order to increase the ion conductivity of the ion exchange membrane 1, hydrogen humidified by a method of passing warm water heated to around 90 ° C. is used for the fuel gas g. In some cases, the oxidizing gas o may be humidified. When the humidified fuel gas g and oxidizing gas o are sent into the cell, the separator 5 is exposed to a high humidity atmosphere. SO ₄ ²⁻ , F ^{− and the} like generated by decomposition of the resin component of the ion exchange membrane 1 may adhere to the surface of the separator 5. As a result, the separator 5 is placed in a corrosive atmosphere in which corrosion and elution are likely to occur. When corrosion and elution occur, the metal ions dissolved from the separator 5 promote the decomposition of the ion exchange membrane 1 or the catalyst in the electrodes 2 and 3 is contaminated, so that the output and durability of the fuel cell are reduced. .

Since the separator material is required to have chemical stability due to the performance of the fuel cell, a carbon block formed into a predetermined shape by cutting or forming, a carbon resin formed by compression, or the like has been conventionally used. However, the processing cost is high and it is difficult to reduce the thickness required for reducing the weight of the fuel cell. Therefore, it has been studied to use stainless steel that can be molded into a predetermined shape as a separator material for fuel cells (Patent Documents 1 and 2).
Japanese Patent No. 3097690 Japanese Patent No. 3269479

  Stainless steel proposed as a separator for fuel cells uses Cr and Mo as main corrosion resistance improving elements, and Cr: 10.5 to 35% by mass, Mo: 0.2 to 6.0% by mass The addition amount is selected. Ti, Nb, etc. may be added. However, high corrosion resistant steels such as SUS436 and SUS444 to which Cr, Mo, Ti, Nb, etc. are added have good corrosion resistance in acid environments, but metal elements are eluted with the progress of corrosion, so the corrosion resistance in separator environments is Not necessarily enough. In fact, in fuel cells incorporating high corrosion resistant steel such as SUS436 and SUS444 as separators, metal ions are leached and output tends to decrease early. Among them, Fe that is most contained in the stainless steel plate is eluted as Fe ions and promotes the decomposition of the ion exchange membrane.

Among ferritic stainless steels, SUS447J1 is listed as the steel type with the most excellent corrosion resistance. SUS447J1 containing 30Cr-2Mo as a basic component is excellent in corrosion resistance and has much less metal ion elution than other steel types. However, SUS447J1 shows higher contact resistance because the steel plate surface is covered with a chromium-based passive film (mixed film of oxide and hydroxide) that is more stable than other ferritic stainless steels. In addition, if the passive film is non-uniform in thickness and has defects, the contact resistance is low but the elution resistance is poor, so the eluted metal ions decompose the ion exchange membrane, and the battery performance improves as the usage time elapses. to degrade. In addition, in order to improve the output of fuel cells, stainless steel for separators is required to have a surface contact resistance of ²⁰ mΩ · cm ² or less, but is required for high corrosion resistance steels such as SUS436 and SUS444, which have high surface contact resistance. In order to obtain a battery output, the number of cell stacks must be increased, and an increase in cost, weight, etc. cannot be avoided.

Therefore, if ferritic stainless steel with SUS447J1 class elution resistance and low contact resistance can be applied to fuel cell separators in a solid state, cost reduction and weight reduction will be achieved, and the practical application of fuel cells will greatly advance. Conceivable.
The present invention is based on knowledge found as a result of investigating and examining the effects of alloy components on corrosion and metal ion elution in the separator environment, and by selecting a ferritic stainless steel having a specific composition as a separator material, An object is to provide a low-cost fuel cell with low surface contact resistance and improved durability.

The ferritic stainless steel for the polymer electrolyte fuel cell separator of the present invention has C: 0.020 mass% or less, Si: 0.50 mass% or less, Mn: 0.50 mass% or less, P: 0.053 to 0.080 mass%, S: 0.005 mass% or less, Ni: 0.50 mass% or less, Cr: 28-32 mass%, Mo: 1.5-2.5 mass%, Cu: 0.80 mass% % To 2.0% by mass or less, Nb: 0.03 to 0.25% by mass, Ti: 0.03 to 0.25% by mass, Al: 0.04 to 0.20% by mass, Nb: 0.0%. 10 to 0.25% by mass, the balance is Fe and inevitable impurities , and C + N is regulated to 0.025% by mass or less.
(The following margin)

  In the ferritic stainless steel for the polymer electrolyte fuel cell separator of the present invention, the elution resistance is improved by appropriately managing the Cr and Mo contents, and further, the stainless steel is obtained by adding the appropriate amount of Cu and strictly controlling the P content. The surface contact resistance is reduced while maintaining the original corrosion resistance. Therefore, it becomes a fuel cell separator with much better processability and productivity than graphite separators, and there is little internal loss due to surface contact resistance even when multiple single cells are stacked, and fuel cell with high power generation efficiency Is obtained. In particular, since it has elution resistance at a low level not found in conventional ferritic stainless steel, it is also suitable as a stationary fuel cell separator for home use with a total operation time of tens of thousands of hours.

  Although the passive film on the surface of the stainless steel plate is effective for the development of corrosion resistance, it is a cause of increasing the surface contact resistance because it is composed of a mixture of oxide and hydroxide with high specific electrical resistance. Moreover, although a strong passive film is advantageous for corrosion resistance, it further increases the surface contact resistance, so it can be said that the improvement in corrosion resistance and the reduction in surface contact resistance are contradictory. Therefore, the present inventors first examined an optimum component system that suppresses elution of metal ions, and further investigated and studied a method for reducing surface contact resistance without significantly degrading the elution resistance by adding a third element. .

In a high humidity and acid environment in the cell, a stainless steel plate to which appropriate amounts of Cr and Mo are added exhibits excellent elution resistance. Therefore, as a result of detailed investigation and examination of the effects of the addition of the third element in the component system to which Cr and Mo were added, Cu exceeding the level contained in normal ferritic stainless steel was added, and the P content was appropriate It has been found that the surface contact resistance decreases when it is controlled. Although the influence of Cu addition and P content control on the decrease in surface contact resistance is not necessarily clear, it is presumed that the following mechanism is effective.
When Cu is added at a level contained in ordinary ferritic stainless steel (specifically, an amount exceeding 0.80% by mass), the Cu concentration of the passive film increases according to the Cu content, and the passive film Is given conductivity or semiconducting properties. P is concentrated on the surface of the stainless steel used in the fuel cell separator, and suppresses the elution of metal ions by reattaching P eluted in the acidic environment in the cell or generating a P compound. Incorporation of the compound improves the conductivity of the passive film.

Hereinafter, the alloy components, contents, and the like of the ferritic stainless steel used in the fuel cell separator of the present invention will be described.
[C, N: 0.020 mass% or less]
C and N are components that lower the workability and low-temperature toughness of ferritic stainless steel. In this component system containing a large amount of Cr and Mo, C and N are reduced as much as possible to ensure workability and low-temperature toughness. It is preferable to do. For this reason, the upper limit of the C and N content is specified as 0.020 mass%, and the total content of C and N is regulated to 0.025 mass% or less. In order to secure a higher level of workability and low temperature toughness, the total content of C and N is preferably regulated to 0.020% by mass or less.
[Si, Mn: 0.50 mass% or less]
Since Si hardens ferritic stainless steel and Mn reduces corrosion resistance, the lower the content of both Si and Mn, the more preferably the upper limit of the content is regulated to 0.50% by mass.

[P: 0.020 to 0.080 mass%]
It is an extremely effective component for improving corrosion resistance, overall corrosion resistance, and dissolution resistance in high humidity and acidic environments to which fuel cell separators are exposed, and is also effective in reducing surface contact resistance. The effect of addition of P is observed at 0.020% by mass or more, but becomes more effective at 0.030% by mass or more. However, since the stainless steel becomes harder and the workability is hindered with an increase in the amount of P added, the upper limit of the P content is specified as 0.80% by mass. In order to ensure workability in addition to dissolution resistance and low surface contact resistance, the P content is selected in the range of 0.030 to 0.060 mass%.

[S: 0.005 mass% or less]
Since it adversely affects the corrosion resistance of stainless steel, it is preferable to reduce it as much as possible. In this component system, the upper limit of the S content is defined as 0.005% by mass.
[Ni: 0.50 mass% or less]
Since a large amount of Ni deteriorates the elution resistance, the upper limit of the content is regulated to 0.50% by mass.
[Cu: more than 0.80% by mass and 2.0% by mass or less]
It is an effective component for reducing the surface contact resistance, and an effect of adding Cu is seen in an amount exceeding 0.80% by mass. However, excessive addition impairs dissolution resistance, so the upper limit of Cu content was set to 2.0 mass%. By adding Cu, the low temperature toughness of the ferritic stainless steel is also improved. The Cu content is preferably selected in the range of 1.00 to 1.70% by mass.

[Cr: 28-32% by mass]
In order to ensure corrosion resistance in the separator environment, a Cr content of 28% by mass or more is necessary. Corrosion resistance improves with an increase in Cr content, but excessive addition deteriorates workability and low temperature toughness, so the upper limit of Cr content was regulated to 32 mass%.
[Mo: 1.5 to 2.5% by mass]
In order to ensure corrosion resistance in the separator environment, 1.5% by mass or more of Mo is necessary, but excessive addition hardens stainless steel, so the upper limit of the Mo content was regulated to 2.5% by mass.

[Nb, Ti: 0.03 to 0.25% by mass]
It is an effective component for improving the corrosion resistance and workability of the weld zone. In this component system, C is fixed by Nb and N is fixed by Ti. The action of fixing C and N is observed when Nb and Ti are added in an amount of 0.03 mass% or more, but excessive addition exceeding 0.25 mass% adversely affects workability and low temperature toughness.
[Al: 0.04 to 0.20 mass%]
In addition to the combined addition of Nb and Ti, the addition of Al stabilizes the fixation of N and further improves the corrosion resistance of the weld. The fixing effect of N is observed when Al is added in an amount of 0.04% by mass or more. However, in this component system containing a large amount of P, it is difficult to ensure low-temperature toughness if an excessive amount of Al exceeding 0.20% by mass is added. .

[Other ingredients]
It is possible to add other components to the ferritic stainless steel used in the present invention as long as the production cost, corrosion resistance, and contact resistance are not significantly impaired. For example, the corrosion resistance of the welded portion is such that carbonitride-generating elements such as V, Ta, Zr, and Hf are in the range of 0.1 to 0.25% by mass, or sulfide-generating elements such as Mg, Ca, and Y are used. When added in the range of 0.1% by mass or less, it is improved. W, Co, Sn of 0.50 mass% or less effective for improving corrosion resistance and 0.1 mass% or less of B effective for improving low temperature toughness are also effective alloy components.

  Ferritic stainless steel with the components shown in Table 1 is melted in a vacuum melting furnace, and after casting and hot rolling, annealing, pickling, and cold rolling are repeated to produce a cold-rolled annealed sheet with a thickness of 0.1 mm. did.

Carbon paper was brought into contact with the surface of a 100 mm square test piece cut out from each cold-rolled annealed plate, and the resistance value when a surface pressure of 15 kgf / cm ² was applied was measured by the four probe method. The measured resistance value was R (mΩ), the cross-sectional area of the test piece was S (cm ² ), and the contact resistance ρ ′ (mΩ · cm ² ) was calculated according to the formula ρ ′ = R × S (mΩ · cm ² ).
Next, the cold-rolled annealed plate was pressed to produce a separator having a gas flow path with a height of 0.4 mm. The separator was assembled in a single cell, and the condensed water obtained by the battery reaction was circulated and re-supplied to the fuel electrode side. A fuel cell using a fluororesin was assembled in the piping and pure water container other than the separator. The fuel cell was generated using hydrogen with a dew point of 90 ° C. as the fuel gas and air as the oxidizing gas.

For humidification of the fuel gas, hydrogen gas was fed into a humidifier with a capacity of 10 liters containing pure water, and pure water reduced by evaporation or the like was replenished every 500 hours. All pure water was replaced in 2500 hours, and metal ions contained in the pure water were analyzed by ICP-MASS method. The elution amount of metal ions was evaluated by the sum of the five elements of Fe, Cr, Ni, Mo, and Cu.
In the constant current operation of 0.5 A / cm ² , the initial battery output was 0.58 to 0.60 V in all cases. During the continuous operation for 5000 hours, the output voltage was measured every 2500 hours, and the output voltage reduction degree was calculated as output voltage reduction degree (%) = (voltage at each measurement point / initial voltage) × 100. Moreover, the corrosion state of the separator taken out from the fuel cell after completion of the test was observed.

As can be seen from the test results in Table 2, the stainless steel separator that satisfies the component conditions defined in the present invention has a significantly lower contact resistance than the separator made from the comparative steel F9 having a low P and Cu content. It can be seen that the number of stacks of cells for extracting necessary power can be reduced. In addition, the elution amount of metal ions was suppressed to a low level even after operation for 2500 hours. The separator after operation for 2500 hours and 5000 hours was very slightly corroded, and the output voltage was hardly lowered.
On the other hand, the separators made from the comparative steels F1, F3, F5, and F7 had insufficient elution resistance and red rust, and the durability required for the fuel cell separator was insufficient.
As is apparent from this comparison, when ferritic stainless steel satisfying the component conditions defined in the present invention is used for the fuel cell separator material, the output voltage is maintained at a high level, and the durability of the fuel cell is improved.

  As described above, when ferritic stainless steel of the specified component design is used for the separator material, low contact resistance is maintained over a long period of time even when exposed to an acidic and high-humidity cell atmosphere, and ion exchange is performed. A fuel cell separator in which elution of metal ions that contaminate the membrane is suppressed can be obtained. Therefore, there is provided a fuel cell with little decrease in output voltage and excellent durability. Moreover, since the stainless steel separator is easier to process than graphite, the fuel cell can be made thinner and lighter.

Sectional view (a) and exploded perspective view (b) illustrating the internal structure of a fuel cell using a solid polymer membrane as the electrolyte

Explanation of symbols

1: Ion exchange membrane 2: Oxidizing electrode 3: Fuel electrode 4: Gasket 5: Separator 5g: Groove 6: Oxidizing gas supply port 7: Oxidizing gas discharge port 8: Fuel gas supply port 9: Fuel gas discharge port g: Fuel gas o: Oxidizing gas

Claims (2)

C: 0.020 mass% or less, Si: 0.50 mass% or less, Mn: 0.50 mass% or less, P: 0.053 to 0.080 mass%, S: 0.005 mass% or less, Ni: 0.50 mass% or less, Cr: 28-32 mass%, Mo: 1.5-2.5 mass%, Cu: more than 0.80 mass% and 2.0 mass% or less, Nb: 0.03-0 .25% by mass, Ti: 0.03 to 0.25% by mass, Al: 0.04 to 0.20% by mass, N: 0.020% by mass or less, the balance being the composition of Fe and inevitable impurities , C + N: Ferritic stainless steel for polymer electrolyte fuel cell separator, characterized by being regulated to 0.025% by mass or less.   A polymer electrolyte fuel cell separator using the ferritic stainless steel according to claim 1 as a material.

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