KR101239476B1 - stainless steel for bipolar plate of PEMFC and method of manufacturing the same - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless steel for fuel cell separator and a method for manufacturing the same, and more particularly, to a stainless steel for polymer fuel cell separator and a surface modification method having a low interfacial contact resistance and excellent formula stability.
The present invention is in weight percent, C: greater than 0 to 0.02, N: greater than 0 to 0.02, Si: greater than 0 to 0.4, Mn: greater than 0 to 0.2, P: greater than 0 to 0.04, S: greater than 0 to 0.02, Cr : 25 to 34, Mo: less than 0 to 0.1, Cu: 0 to 1, Ni: less than 0 to 0.2, Ti: greater than 0 to 0.5, Nb: greater than 0 to 0.5, V: 0.2 to 1, W: 0 The composition and the balance of the single or mixed addition of ~ 4 include Fe and other unavoidable impurities, and provide a stainless steel for polymer fuel cell separators having an interface contact resistance of 10 mΩcm ² or less and excellent formula stability.

Description

Stainless steel for bipolar fuel cell separator and its manufacturing method {stainless steel for bipolar plate of PEMFC and method of manufacturing the same}

The present invention relates to a stainless steel for a polymer fuel cell separator and a method for manufacturing the same, and more particularly, to a stainless steel for a polymer fuel cell separator and a method for manufacturing the same having low interfacial contact resistance and excellent formula stability.

In general, a fuel cell is a power generation device that generates electrical energy from hydrogen energy, and its importance is emphasized as an alternative to conventional fossil fuels. The fuel cell is capable of generating high efficiency compared to the existing power generation device, has little noise and vibration, and is an environmentally friendly, energy-free energy technology with little pollution emission factor, and can be applied from the small scale to the large scale power generation system.

These fuel cells are classified into solid polymer fuel cells, solid oxide fuel cells, phosphate fuel cells, molten carbonate fuel cells, direct methanol fuel cells, and alkaline fuel cells according to the type of electrolyte. Is different.

Among them, the solid polymer fuel cell is a fuel cell using a polymer membrane having hydrogen ion exchange characteristics as an electrolyte, and has a low operating temperature and high efficiency at about 80 ° C. compared with other fuel cells. Also, it has high current density and power density, simple structure, fast starting and response characteristics, and can be used as a power source for automobiles and households.

The main components of such a solid polymer fuel cell consist of a polymer electrolyte membrane, an electrode, and a separator to form a stack. In the solid polymer fuel cell, the separator plate is formed so as to block hydrogen gas and oxygen gas as reaction gases from each other, to electrically connect membrane electrode assembly (MEA), to support the membrane electrode assembly, And maintains the shape. Therefore, it should have a dense structure such that the two gases are not mixed, and should have good electrical conductivity for the role of the conductor and sufficient mechanical strength for the role of the support.

The separator is generally formed of one of graphite, carbon, Ti alloy, stainless steel and conductive plastic, and may preferably be formed of stainless steel. Stainless steel has the advantage of being able to achieve thermal conductivity, low gas permeability, and large area, and can form a good product and be made into a thin piece, thereby reducing the volume and weight of the fuel cell stack.

However, when a stainless steel separator is used, there is a problem that corrosion occurs in an acidic electrolytic solution to contaminate electrodes and electrolytes, and the interface contact resistance increases due to corrosion products on the surface.

The present invention is to solve the problems of the prior art described above and to meet the demands of the new separator material, to ensure a low interfacial contact resistance and at the same time to increase the corrosion resistance stainless steel for fuel cell separator plate and its It is to provide a manufacturing method.

Stainless steel according to the present invention for achieving the above object, in weight%, C: greater than 0 to 0.02, N: greater than 0 to 0.02, Si: greater than 0 to 0.4, Mn: greater than 0 to 0.2, P: greater than 0 ~ 0.04, S: over 0 to 0.02, Cr: 25 to 34, Mo: below 0 to 0.1, Cu: 0 to 1, Ni: below 0 to 0.2, Ti: above 0 to 0.5, Nb: above 0 to 0.5 Single or mixed composition of V: 0.2 ~ 1, W: 0 ~ 4 in the composition and the balance contains Fe and other unavoidable impurities, and the interface contact resistance is 10mΩcm ² or less, and has excellent formula stability. To provide.

Moreover, according to this invention, the Cr / Fe atomic ratio of the said stainless steel for separators is 0.4 or more in 2 nm area | region from the surface.

In addition, according to the present invention, the polymer fuel cell is separated from the stainless steel steel having the composition described above in a solution having a composition of 2 to 10 wt% hydrochloric acid, 2 to 10 wt% hydrofluoric acid, and 1 to 5 wt% hydrogen peroxide. Provided is a method of manufacturing stainless steel for a plate.

In the present invention, the solution is maintained at a temperature of room temperature ~ 60 ℃.

According to the present invention, a low interfacial contact resistance can be ensured by modifying the surface passivation film without severe erosion or formulation of stainless steel.

In addition, according to the present invention, it is possible to produce stainless steel for polymer fuel cell separator having excellent corrosion resistance without additional expensive precious metal coating process.

1A to 1B are photographs of SEM observation of the surface of stainless steel after being immersed in a 15 wt% hydrochloric acid solution at 50 ° C. for 5 minutes.
FIG. 2A is a SEM photograph of the surface of an unmodified stainless steel, and FIG. 2B is a view of stainless steel after being immersed in a mixed solution of 2 wt% hydrochloric acid, 3 wt% hydrofluoric acid, and 2 wt% hydrogen peroxide at 50 ° C. for 1 minute. The photograph which observed the surface by SEM.
3 is a graph showing the Cr / Fe ratio in the passivation film after surface modification according to the invention and comparative examples.

Hereinafter, with reference to the drawings showing an embodiment of the present invention will be described in detail a stainless steel for a fuel cell separator according to the present invention and a manufacturing method thereof.

In the stainless steel for polymer fuel cell separator, a method of obtaining surface characteristics to a desired level is known by improving surface roughness or forming a micro pit for the entire surface for excellent corrosion resistance and low contact resistance. In addition, a method of reducing the contact resistance by controlling the Cr / Fe atomic ratio has been proposed. However, the above methods still require additional processes to supplement the problem of corrosion resistance. In addition, hydrochloric acid and hydrofluoric acid alone immersion to reduce the interfacial contact resistance of the stainless steel for fuel cell separator plate may damage the base material, and may cause corrosion resistance due to erosion and the formation of a formula on the surface. Therefore, a suitable surface modification method with low interfacial contact resistance and no surface damage is desirable.

To this end, the present invention describes a stainless steel material having a low interfacial contact resistance and excellent corrosion resistance for the purpose of a polymer fuel cell separator and a manufacturing method thereof. First, in the present invention, surface modification treatment is performed by maintaining an appropriate time at a temperature of room temperature to 60 ° C. in a solution having a composition of 2 to 10 wt% hydrochloric acid, 2 to 10 wt% hydrofluoric acid, and 1 to 5 wt% hydrogen peroxide. do. Accordingly, the surface passivation film is reformed without erosion or formula on the surface of stainless steel, and the Cr / Fe ratio in the passivation film is 0.4 or more, and Cr concentration is about twice that of the base material Cr / Fe ratio. The contact resistance can ensure 10 mPacm <2> or less.

Hereinafter, a stainless steel material having low interfacial contact resistance and excellent corrosion resistance for the purpose of a polymer fuel cell separator will be described in more detail.

The stainless steel of the present embodiment is in weight%, C: greater than 0 to 0.02, N: greater than 0 to 0.02, Si: greater than 0 to 0.4, Mn: greater than 0 to 0.2, P: greater than 0 to 0.04, S: greater than 0 to 0.02, Cr: 25-34, Mo: 0-0.1 or less, Cu: 0-1, Ni: 0-0.2, Ti: 0-0.5, Nb: 0-0.5, V: 0.2-1, W: 0 ~ 4 single or mixed addition and balance are produced by steelmaking, refining and continuous casting of steel materials containing Fe and other unavoidable impurities, and hot rolled, annealed, pickled, cold rolled and polished. The annealing may be repeated to produce a cold rolled annealing plate or coil having a thickness of 0.075 mm to 2 mm. Table 1 shows the composition of the manufactured stainless steel.

Steel grade C Si Mn P S Al Cr Ni Cu Ti Nb Mo Etc N Example 1 0.006 0.141 0.152 <0.003 <0.003 0.051 30.25 0.11 1.00 0.049 0.26 - 0.39V 0.011 Example 2 0.008 0.110 0.151 <0.003 <0.003 0.083 30.19 0.11 0.98 0.056 0.26 - 0.37 W 0.009

Hereinafter, the reason for limiting each component content will be described in detail. In addition, all% described below is weight%.

C and N form Cr carbonitride in stainless steel, and as a result, the corrosion resistance of the Cr-deficient layer is lowered, so the lower the content of the two elements is preferable. In the present invention, the composition ratio is limited to C: 0.02% or less and N: 0.02% or less.

Since Si suppresses the element, toughness, and moldability which are effective for deoxidation, in the present invention, the composition ratio of Si is limited to within 0.4%.

Mn is an element that increases deoxidation, but the inclusion MnS reduces corrosion resistance. In the present invention, the composition ratio of Mn is limited to within 0.2%.

Since P not only reduces corrosion resistance but also toughness, in this embodiment, the composition ratio of P is limited to within 0.04%.

S forms MnS, which becomes a starting point of corrosion and reduces corrosion resistance. In this embodiment, the composition ratio of S is limited to within 0.02% in consideration of this.

Cr increases the corrosion resistance in the acidic atmosphere in which the fuel cell is operated, but reduces the toughness, and therefore, in this embodiment, the appropriate composition ratio of Cr is limited to 25% to 34%.

Since Mo can degrade the economics and toughness of the material, in the present embodiment, the composition ratio of Mo is limited to the range of 0% to less than 0.1%.

Cu increases the corrosion resistance in an acidic atmosphere in which the fuel cell operates, but the performance of the fuel cell may be degraded due to the elution of Cu when excessively added. In this embodiment, the composition ratio of Cu is limited within 0% to 1% in consideration of this.

When Ni is excessively added, Ni elution and moldability may decrease. In this embodiment, the composition ratio of Ni is limited to less than 0.2% in consideration of this.

Ti and Nb deteriorate the element and toughness effective for forming C and N in carbon as carbonitrides. In this embodiment, each composition ratio is limited to 0.5% or less in consideration of this.

In addition to this, one or two or more kinds of V and W may be added, and their composition ratios are as follows.

V increases the corrosion resistance in the acidic atmosphere in which the fuel cell operates, but when excessively added, ions are eluted and the performance of the cell may be degraded. In this embodiment, the composition ratio of V is limited to 0.2 to 1% in consideration of this.

W has the effect of increasing the corrosion resistance and lowering the interfacial contact resistance in the acidic atmosphere in which the fuel cell operates, but deteriorates the toughness upon excessive addition. In this embodiment, the composition ratio of W is limited to 4% or less in consideration of this.

Hereinafter, the surface modification treatment conditions of stainless steel for low interfacial contact resistance and excellent corrosion resistance will be described in more detail.

Although the interfacial contact resistance of stainless steel shows a lower value as the average roughness of the surface becomes rough, it is difficult to secure an interfacial contact resistance of 10 mΩcm 2 or less, which is known to be suitable as a separator material by simply adjusting the surface roughness. This is because stainless steel combines with oxygen in the air in its natural state to form a thin protective passivation film on its surface. Therefore, it is necessary to modify the passivation film existing on the stainless steel surface. In addition, since the passivation film formed after annealing or in the air has a high Fe content, it must be reformed into a passivation film containing Cr by increasing the ratio of Cr to be used for a fuel cell separator. The reason is that Cr oxide is more stable than Fe oxide and has excellent corrosion resistance, and thus is more suitable as a surface oxide of fuel cell separators requiring high corrosion resistance.

Therefore, in the present invention, the surface modification treatment was performed by maintaining a proper time at a temperature of room temperature to 60 ℃ in a solution having a composition of 2 to 10% by weight hydrochloric acid, 2 to 10% by weight hydrofluoric acid, and 1 to 5% by weight hydrogen peroxide. When only one of hydrochloric acid and hydrofluoric acid is used in surface modification, it is difficult to maintain the reforming continuity of the solution, and the reforming continuity is maintained when repeated surface modification is performed by mixing an appropriate amount of hydrochloric acid and hydrofluoric acid.

In the present invention, the concentration of hydrochloric acid and hydrofluoric acid is limited to 2% by weight to 10% by weight, but less than 2% by weight of the surface modification effect is small, on the contrary too high may cause damage to the base material and formula.

Since hydrochloric acid and hydrofluoric acid serve to remove the passivation film present on the stainless steel surface during immersion, the passivation film having a high Fe content in the atmosphere is again generated after immersion. In this case, an additional process is required to form a passivation film having a high Cr content, and there is a problem of increasing production cost in terms of economy. Therefore, by adding an oxidizing agent such as hydrogen peroxide to the mixed acid of hydrochloric acid and hydrofluoric acid, it is possible to make a passivation film in which Cr oxide is concentrated on the surface and to reduce the production cost because no additional process is required.

Oxidizers, such as hydrogen peroxide, accelerate the dissolution rate of stainless steel to promote oxide formation on the surface of stainless steel. Since Cr has a greater bonding force with oxygen than Fe, it is possible to form a passivation film in which Cr oxide is concentrated on the surface by the addition of hydrogen peroxide. Hydrogen peroxide can also prevent erosion and formulation of stainless steel by hydrochloric acid and hydrofluoric acid.

In the present invention, the hydrogen peroxide concentration is limited to 1% by weight to 5% by weight, but if less than 1% by weight, the passivation film may become unstable and erosion and formulation may occur due to hydrochloric acid and hydrofluoric acid. And inadequate formula prevention effect, which is uneconomical and may result in lowering of the rate of surface modification.

Table 2 shows the results of measuring the interfacial contact resistance and formula stability according to Inventive Examples 1 to 6 and Comparative Examples 1 to 11 of the present invention while varying the surface modification conditions (immersion solution, temperature, time). .

Specifically, in the case of Comparative Examples 1 and 2, the solution was immersed in hydrofluoric acid solution at room temperature, and Comparative Examples 3 and 4 were immersed in hydrochloric acid solution at room temperature, and Comparative Examples 5 to 7, 10 and Examples 1 to 6 In this case, the solution was immersed in a mixture of hydrochloric acid and hydrofluoric acid and added hydrogen peroxide at room temperature and 50 ° C. In Comparative Examples 8 and 9, FeCl 3 as an oxidizing agent was added to a mixed solution of hydrochloric acid, hydrofluoric acid and hydrogen peroxide. Is the interfacial contact resistance measured without surface modification.

division Surface Modification Solution Composition (Unit: Weight%) Immersion temperature
(℃) Immersion time Interface contact resistance
(mΩcm ² )
at 150N / cm ² Formula stability HCl HF H2O2 FeCl3 Comparative Example 1 - 2 - - Room temperature 30 seconds 28.0 ○ Comparative Example 2 - 15 - - Room temperature 3 minutes 3.4 X Comparative Example 3 10 - - - Room temperature 5 minutes 4.2 X Comparative Example 4 15 - - - Room temperature 5 minutes 3.5 X Comparative Example 5 One 2 One - Room temperature 5 minutes 26.3 ○ Comparative Example 6 One 2 One - Room temperature 10 minutes 20.1 ○ Comparative Example 7 One 2 One - 50 5 minutes 13.3 ○ Inventory 1 2 4 2 - Room temperature 1 minute 9.6 ○ Inventory 2 2 4 2 - 40 1 minute 8.4 ○ Inventory 3 2 4 2 - 50 1 minute 5.9 △ Honorable 4 2 4 2 - 60 1 minute 5.5 △ Inventory 5 2 4 2 - 50 30 seconds 6.0 ○ Inventory 6 2 4 2 - 50 2 minutes 6.5 △ Comparative Example 8 2 4 2 10 50 1 minute 21.5 ○ Comparative Example 9 2 4 2 10 50 2 minutes 22.1 ○ Comparative Example 10 5 7 5 - 50 1 minute 4.3 X Comparative Example 11 - - - - - - 32.5 ○

In Table 2, the evaluation criteria for the formula stability of the stainless steel is represented by △ if a formula of 1 μm or more on the surface after surface modification, △ if a fine formula of 1 μm or less is present, and ○ if no formula is present on the surface. After surface modification, corrosion resistance may occur due to erosion and the occurrence of a formula on the surface of stainless steel, so it is important that the surface is eroded and formulated after surface modification.

As shown in Table 2, even when the temperature of the reforming solution is room temperature, when the composition of hydrochloric acid and hydrofluoric acid is 10% by weight or more, low interfacial contact resistance could be obtained. In this case, however, the surface is severely damaged and a large formula of 1 μm or more is generated locally, which may be rather detrimental to corrosion resistance.

1A to 1B are SEM photographs of the surface of stainless steel after immersion in 15 wt% hydrochloric acid solution at 50 ° C. for 5 minutes, and FIG. 2A is a SEM photograph of the surface of stainless steel without surface modification. FIG. 2B is a photograph showing an SEM observation of the surface of stainless steel after immersion in a mixed solution of 2 wt% hydrochloric acid, 3 wt% hydrofluoric acid, and 2 wt% hydrogen peroxide at 50 ° C. for 1 minute. 3 is a graph showing the Cr / Fe ratio in the passivation film after surface modification according to the invention and comparative examples.

1 is a SEM photograph of the surface of the modified stainless steel, which was immersed in a hydrochloric acid solution at 15 wt% and 50 ° C. for 5 minutes according to the comparative example of Table 2. Referring to FIG. 1, the surface is completely damaged, and a local formula is also observed. For proper surface modification, hydrochloric acid and hydrofluoric acid should be limited to a composition of 10 wt% or less, and in order to facilitate surface modification, it can be seen that a steel having excellent interfacial contact resistance can be obtained at a solution temperature of 50 ° C.

As can be seen from Comparative Examples 5 to 7, when the composition of hydrochloric acid and hydrofluoric acid is 2% by weight or less, surface modification is not appropriate, and thus it is understood that there is no effect of reducing interfacial contact resistance. As shown in Comparative Examples 8 to 9, it was found that the interfacial contact resistance increased by increasing the thickness of the passivation film when excessive addition of the oxidizing agent FeCl ₃ . Therefore, the addition of an appropriate amount of hydrogen peroxide, which is an oxidizing agent, increases the Cr / Fe ratio in the passivation film and prevents stainless steel erosion and formulation by hydrochloric acid and hydrofluoric acid.

FIG. 2A is an SEM photograph of the surface of Comparative Example 11 of Table 2, and FIG. 2B is an SEM photograph of the surface of Inventive Example 3 of Table 2. FIG. Unlike FIGS. 1A-1B, the surface of FIG. 2B is damaged and shows no erosion or local corrosion.

FIG. 3 shows the Cr / Fe ratio of the passivation film on the surface of stainless steel after surface modification for Comparative Example 11 (the same as before surface modification), Comparative Example 4 and Inventive Example 3 of Table 2. FIG. Comparative Example 4, which is a surface modification method by dipping hydrochloric acid alone, showed almost the same Cr / Fe ratio as Comparative Example 11 before surface modification. However, as seen in Inventive Example 3, it can be seen that the Cr / Fe ratio in the passivation film increased by more than two times compared to 0.35 of the base material due to the addition of the oxidizing agent.

In the above experiments, in the surface modification method of the mixed solution of hydrochloric acid, hydrofluoric acid and hydrogen peroxide of the stainless steel having the composition range of the present invention, it can be seen that the conditions such as the temperature, the composition of the solution acts as an important factor in reducing the interfacial contact resistance there was.

According to the present invention, by modifying the passivation film of stainless steel, low interfacial contact resistance can be ensured, and after the surface modification by non-oxidizing acid, high Cr / Fe is added by addition of an oxidizing agent without additional passivation film formation process. The excellent corrosion resistance of the ratio makes it possible to produce stainless steel suitable for polymer fuel cell separators.

In the above-described invention, the polymer fuel cell separator is described as an example, but it can be applied to various other fuel cell separators. Although the technical spirit of the present invention has been described in detail according to the above-described preferred invention, it should be noted that the above-described invention is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various invention examples are possible within the scope of the technical idea of the present invention.

Claims (5)

By weight%, C: more than 0 to 0.02, N: more than 0 to 0.02, Si: more than 0 to 0.4, Mn: more than 0 to 0.2, P: more than 0 to 0.04, S: more than 0 to 0.02, Cr: 25 to 34, Mo: less than 0 ~ 0.1, Cu: 0 ~ 1, Ni: less than 0 ~ 0.2, Ti: more than 0 ~ 0.5, Nb: more than 0 ~ 0.5, remainder consists of Fe and other unavoidable impurities Stainless steel for polymer fuel cell separator with excellent formula stability with less than 10mΩcm ² and Cr / Fe atomic ratio of 0.4 or more in 2nm area from the surface. The method of claim 1,
By weight%, the stainless steel is a stainless steel for a polymer fuel cell separator plate excellent in formula stability with one or more of V: 0.2-1, W: 0-4. The method of claim 2,
The stainless steel is a stainless steel for a polymer fuel cell separator having excellent ferritic formula stability. A polymer having excellent formula stability for surface modification treatment of a stainless steel made of any one of claims 1 to 3 in a solution containing 2 to 10 wt% hydrochloric acid, 2 to 10 wt% hydrofluoric acid, and 1 to 5 wt% hydrogen peroxide. Method for manufacturing stainless steel for fuel cell separator. 5. The method of claim 4,
The solution is a method for producing a stainless steel for polymer fuel cell separator having excellent formula stability maintained at a temperature of room temperature ~ 60 ℃.

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