Stainless steel for fuel cell separator plates with excellent contact resistance and method for producing the same

Abstract

The present invention provides stainless steel with low contact resistance as a fuel cell separator material and surface roughness parameters representative of its surface shape. [Solution] The present invention is characterized in that it contains, by weight, C: more than 0 and not more than 0.02%, N: more than 0 and not more than 0.02%, Si: more than 0 and not more than 0.4%, Mn: more than 0 and not more than 0.3%, P: more than 0 and not more than 0.04%, S: more than 0 and not more than 0.02%, Cr: 15 to 34%, Cu: more than 0 and not more than 1%, Ni: more than 0 and less than 0.4%, at least one of Ti and Nb, the composition ratio of the contained elements being more than 0 and not more than 0.5%, with the remainder being Fe and other unavoidable impurities, and at least one surface has a three-dimensional arithmetic average roughness (Sa) of 0.05 μm or more and a developed interfacial area ratio (Sdr) of 5% or more.

Description

Technical Field

[0001] The present invention relates to a stainless steel for a fuel cell separator plate having excellent contact resistance and a method for manufacturing the same. More specifically, the present invention relates to a stainless steel for a fuel cell separator plate having excellent contact resistance, which can control the shape of the separator plate surface to increase the contact area with a GDL (Gas Diffusion Layer) and ensure a low contact resistance, and a method for manufacturing the same.

Background Art

[0002] Generally, a fuel cell stack is in a form in which cells including an electrolyte, an electrode, an MEA (Membrane Electrode Assembly) including a GDL, and a separator plate are stacked. Therefore, the separator plate is in contact with the GDL, and the performance of the cell and the fuel cell is reduced by the contact resistance, which is the resistance caused at the separator plate / GDL interface.

[0003] The contact resistance of such a separator plate is mainly affected by two factors. First, it is a passive film, which is an oxide layer on the surface of the metal separator plate. The passive film is a method for ensuring high corrosion resistance, but since it is a non-conductive oxide layer in terms of contact resistance, it preferably has as thin a thickness as possible. Second, the contact area between the separator plate and the GDL affects the contact resistance. It is not the apparent contact area between the separator plate and the GDL, but the actual contact area between the two objects that is important. The actual contact area between the separator plate and the GDL is greatly affected by the surface shapes of the separator plate and the GDL, and in particular, it is determined that the fine surface shape below the micron unit existing on the surface has a main influence. Since the contact resistance tends to be low when the actual contact area between the separator plate and the GDL is large and high when the actual contact area is small, the contact resistance reduction effect varies depending on how the surface shape of the separator plate is changed.

[0004] Attempts have been made to reduce contact resistance by adjusting the surface shape. However, although the Patent Documents describe the use of a separator plate made of stainless steel with unevenness formed on the surface, and it is mainly described that the surface roughness parameter: center line average surface roughness (Ra), that is, the arithmetic average surface roughness is preferably 0.03 to 2 μm. However, even stainless steels with similar Ra may have different contact resistance differences. Therefore, there is a disadvantage that it is difficult to predict the change in contact resistance only by the range of Ra, which is a 2D surface parameter.

[0005] Therefore, in order to significantly reduce the contact resistance, it is necessary to understand the relationship between the contact area and the surface parameters, embody a surface shape that can maximize the contact area with the GDL, and define this as a representative surface parameter.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] An object of the present invention is to provide a stainless steel with low contact resistance as a fuel cell separator material and a surface roughness parameter representing its surface shape.

Means for Solving the Problems

[0008] The stainless steel for a fuel cell separator plate with excellent contact resistance of the present invention contains, by weight %, C: more than 0 and 0.02% or less, N: more than 0 and 0.02% or less, Si: more than 0 and 0.4% or less, Mn: more than 0 and 0.3% or less, P: more than 0 and 0.04% or less, S: more than 0 and 0.02% or less, Cr: 15 to 34%, Cu: more than 0 and 1% or less, Ni: more than 0 and less than 0.4%, and at least one of Ti and Nb. The composition ratio of the contained elements is more than 0 and 0.5% or less, and the balance is composed of Fe and other inevitable impurities. At least one surface has a three-dimensional arithmetic mean roughness (Sa) of 0.05 μm or more and a surface area increase ratio (developed interfacial area ratio, Sdr) of 5% or more.

[0009] The manufacturing method of the stainless steel for a fuel cell separator plate with excellent contact resistance of the present invention contains, by weight %, C: more than 0 and 0.02% or less, N: more than 0 and 0.02% or less, Si: more than 0 and 0.4% or less, Mn: more than 0 and 0.3% or less, P: more than 0 and 0.04% or less, S: more than 0 and 0.02% or less, Cr: 15 to 34%, Cu: more than 0 and 1% or less, Ni: more than 0 and less than 0.4%, and at least one of Ti and Nb. The composition ratio of the contained elements is more than 0 and 0.5% or less, and the balance is composed of Fe and other inevitable impurities. At least one surface has a three-dimensional arithmetic mean roughness (Sa) of 0.05 μm or more and a surface area increase ratio (developed interfacial area ratio, Sdr) of 5% or more. The method includes hot rolling and cold rolling the stainless steel to produce a cold-rolled thin sheet, and immersing the cold-rolled thin sheet in an acid solution.

Advantages of the Invention

[0010] According to the manufacturing method of the separator plate of the present invention, a separator plate with low contact resistance can be manufactured without an expensive coating process in a fuel cell environment.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Best Mode for Carrying Out the Invention

[0012] The stainless steel for a fuel cell separator plate with excellent contact resistance of the present invention contains, by weight%, C: more than 0 and not more than 0.02%, N: more than 0 and not more than 0.02%, Si: more than 0 and not more than 0.4%, Mn: more than 0 and not more than 0.3%, P: more than 0 and not more than 0.04%, S: more than 0 and not more than 0.02%, Cr: 15 to 34%, Cu: more than 0 and not more than 1%, Ni: more than 0 and less than 0.4%, and at least one of Ti and Nb. The composition ratio of the contained elements is more than 0 and not more than 0.5%. The balance consists of Fe and other inevitable impurities. At least one surface has a three-dimensional arithmetic mean roughness (Sa) of 0.05 μm or more and a surface area increase ratio (developed interfacial area ratio, Sdr) of 5% or more.

[0013] This specification does not describe all elements of the examples, and contents that are common in the technical field to which the present invention pertains or are duplicated in the examples are omitted. Also, when a certain part “includes” a certain component, this means that, unless otherwise stated to the contrary, it does not exclude other components, but may further include other components.

[0014] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0015] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

[0016] [Stainless Steel for Fuel Cell Separator Plate] The stainless steel for a fuel cell separator plate with excellent contact resistance of the present invention contains, by weight %, C: more than 0 and not more than 0.02%, N: more than 0 and not more than 0.02%, Si: more than 0 and not more than 0.4%, Mn: more than 0 and not more than 0.3%, P: more than 0 and not more than 0.04%, S: more than 0 and not more than 0.02%, Cr: 15 to 34%, Cu: more than 0 and not more than 1%, Ni: more than 0 and less than 0.4%, and at least one of Ti and Nb, the composition ratio of the contained elements is more than 0 and not more than 0.5%, the balance is composed of Fe and other inevitable impurities, and at least one surface has a three-dimensional arithmetic mean roughness (Sa) of 0.05 μm or more and a surface area increase ratio (developed interfacial area ratio, Sdr) of 5% or more.

[0017] The Sa, which is the surface parameter, is the arithmetic mean roughness of the separator plate surface, and the Sdr is the surface area increase ratio generated by the unevenness present on the separator plate surface. The parameter has the advantage that it can more precisely define the shape of the surface with three-dimensional parameters different from the conventional two-dimensional surface parameters. Therefore, the parameter affects the actual contact area between the separator plate and the GDL and acts as a factor for reducing the separator plate contact resistance. The Sa and Sdr parameters are defined according to the ISO 25178 standard. The Sa is the arithmetic mean roughness for a three-dimensional surface different from the Ra, which is the two-dimensional arithmetic mean roughness, and the unit is μm. The Sdr means the ratio increased with respect to the apparent area of the developed area, which is the actual surface area of the measured shape, as the three-dimensional surface area increase ratio, and the unit uses % obtained by multiplying the increase ratio by 100. Since a surface that is a perfect plane has no increased area with respect to the apparent area, the Sdr is 0%, and if there is any shape having a height such as unevenness or wrinkles on the surface, it means that the Sdr has a number greater than 0.

[0018] Therefore, at least one surface of the separator plate according to the present invention has a three-dimensional arithmetic mean roughness Sa of 0.05 μm or more and a developed interfacial area ratio (Sdr) of 5% or more. This means that the actual contact area with the GDL increases due to the presence of irregularities with height depending on the shape of the separator plate surface, and the contact resistance may be significantly reduced.

[0019] According to an embodiment of the present invention, the stainless steel for a fuel cell separator having excellent contact resistance has a contact resistance value of 10 mΩ·cm. 2 Here, the contact resistance means the interfacial contact resistance of one surface of the contact surface between the separation plate surface and the GDL surface.

[0020] Unless otherwise specified below, the unit is % by weight. Furthermore, when a part "contains" a certain component, this does not mean that other components are excluded, but that other components may also be included, unless otherwise specified.

[0021] Carbon (C), Nitrogen (N) Carbon (C) and nitrogen (N) form Cr carbonitrides in steel, which reduces the corrosion resistance of the Cr-deficient layer, so the lower the content of these elements, the better. Therefore, in the present invention, it is preferable to limit the composition ratio of C to more than 0 but not more than 0.02%, and N to more than 0 but not more than 0.02%.

[0022] Silicon (Si) Although Si has the advantage of improving high-temperature oxidation resistance and strengthening the passive film in stainless steel to improve corrosion resistance, excessive addition of Si reduces the elongation rate. Therefore, in the present invention, it is preferable to limit the composition ratio of Si to more than 0% and not more than 0.4%.

[0023] Manganese (Mn) Although Mn is an element that increases deoxidation, MnS inclusions reduce corrosion resistance, so in the present invention, it is preferable to limit the composition ratio of Mn to more than 0% and 0.3% or less.

[0024] Phosphorus (P) Since it reduces not only corrosion resistance but also toughness, in the present invention, it is preferable to limit the composition ratio of P to more than 0 and not more than 0.04%.

[0025] Sulfur (S) MnS is formed, and MnS becomes a starting point of corrosion and reduces corrosion resistance. Therefore, in the present invention, considering this, it is preferable to limit the composition ratio of S to more than 0 and not more than 0.02%.

[0026] Chromium (Cr) It is an element that promotes the formation of stainless steel oxide. For corrosion resistance, it is necessary to add 15% or more of Cr. When added in excess, there is a problem that the Sticking defect increases due to the formation of a dense oxide scale during hot rolling. It is preferable to limit the composition ratio of Cr to 35% or less, and more preferably 34% or less.

[0027] Titanium (Ti), Nb (Niobium) Titanium (Ti) and niobium (Nb) are effective elements for forming carbonitrides with C and N in steel, but they reduce toughness. Therefore, in the present invention, considering this, it contains at least one of Ti and Nb, and the composition ratio of the contained element is preferably limited to more than 0 and not more than 0.5%.

[0028] Copper (Cu) It increases corrosion resistance in the acidic atmosphere in which the fuel cell operates, but when added in excess, the performance and formability of the fuel cell may decrease due to the elution of Cu. Therefore, in the present invention, considering this, it is preferable to limit the composition ratio of Cu to more than 0 and not more than 1%.

[0029] Nickel (Ni) Nickel (Ni) is an impurity inevitably contained in steel. As an element that stabilizes the austenite phase like C and N, it is an element that slows down the corrosion rate and improves corrosion resistance. However, considering the problem of decreased formability and the economy due to high cost when added in excess, it is limited to more than 0 and less than 0.4%.

[0030] [Method for Manufacturing Stainless Steel for Fuel Cell Separator Plate] The method for manufacturing stainless steel for a fuel cell separator plate with excellent contact resistance according to the present invention, in terms of weight %, has 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 0.3%, P: more than 0 to 0.04%, S: more than 0 to 0.02%, Cr: 15 to 34%, Cu: more than 0 to 1%, Ni: more than 0 to less than 0.4%, includes at least one of Ti and Nb, the composition ratio of the contained elements is more than 0 to 0.5%, the balance consists of Fe and other inevitable impurities, and at least one surface has a three-dimensional arithmetic mean roughness (Sa) of 0.05 μm or more and a surface area increase ratio (developed interfacial area ratio, Sdr) of 5% or more. The method includes hot rolling and cold rolling the stainless steel to produce a cold-rolled thin sheet, and immersing the cold-rolled thin sheet in an acid solution.

[0031] Also, the acid solution for immersing the cold-rolled stainless steel sheet of the present invention may contain hydrochloric acid or sulfuric acid.

[0032] Also, the method for manufacturing stainless steel for a fuel cell separator plate with excellent contact resistance according to the present invention, after the step of immersing the cold-rolled stainless steel sheet in an acid solution, is subjected to primary electrolysis treatment at a current density of 0.15 to 0.45 A / cm 2 and then subjected to secondary electrolysis treatment at a current density of 0.03 to 0.07 A / cm 2 and may further include a step of immersing in a mixed acid solution.

[0033] Hereinafter, the present invention will be described more specifically through examples.

[0034] The present invention is not limited only to the following examples and may be embodied in other forms.

[0035] (Examples) Table 1 shows the alloy components of the stainless steel for the fuel cell separator plate. The stainless steel used in the present invention was produced by cold rolling a stainless steel having the above composition at the cold rolling stage using a Z-mil cold rolling machine to produce a cold rolled thin sheet, and then bright annealing heat treatment of the cold rolled thin sheet at the heat treatment stage.

[0036] [Table 1]

[0037] The surface shape of the inventive steel and the comparative steel described in Table 1 was adjusted according to the conditions in Table 2 below, and the relationship between the contact resistance and the surface parameters was examined. The method for adjusting the surface shape of the stainless steel may be manufactured through the following steps. In the surface shape adjustment step, only step A may be performed, or steps A and D, steps B and D may be performed in sequence, or steps C, C, and D may be performed in sequence. The corrosion resistance in the fuel cell environment before and after passing through the surface shape adjustment step was evaluated. The corrosion resistance evaluation was carried out by measuring the current density at 0.6 V with respect to the saturated calomel electrode (SCE) which is the reference electrode after an anodic polarization experiment at 80 °C in a mixed solution of 0.05 M sulfuric acid and 2 ppm hydrofluoric acid which is the fuel cell operating environment after the surface shape adjustment step. When the current density is 1.0 μA / cm 2 The following was considered good, and exceeding 1.0 μA / cm 2 was judged as bad.

[0038] The contact resistance evaluation was measured after passing through the surface shape adjustment step. Two prepared cold rolled materials were prepared, and carbon paper (SGL-10BA) was placed between them. After evaluating the interfacial contact resistance 4 times at a contact pressure of 100 N / cm 2 the average value was calculated.

[0039] [Table 2]

[0040] Referring to the results in Table 2, Examples 1 to 13 satisfy Sa of 0.05 μm or more and Sdr value of 5% or more according to the present invention, and thus 10 mΩ·cm2 It can be seen that it has the following low contact resistance. Also, in the corrosion resistance evaluation in a fuel cell environment, it can be confirmed that the corrosion resistance is good at 1.0 μA / cm 2 or less.

[0041] On the other hand, in Comparative Example 1 and Comparative Example 2, it is possible to secure a low contact resistance of 10 mΩ·cm or less through the surface shape adjustment process, but the corrosion resistance was poor due to the low Cr content of Comparative Steel 1. Therefore, since the corrosion resistance as a fuel cell separator material can greatly affect the durability of the fuel cell, it can be seen that materials with poor corrosion resistance are difficult to apply as separator materials. 2 Moreover, Examples 4 to 7 to which mixed acid immersion was added showed higher Sa and Sdr values compared to Examples 1 to 3 in which only simple hydrochloric acid immersion was performed. This is presumably because the dissolution of the surface was more actively carried out through mixed acid immersion, and through this, many small irregularities were generated on the surface.

[0042]

[0043] Also, referring to the results of Examples 8 to 13, it can be seen that surface shape adjustment is more effective by applying sulfuric acid electrolysis. Through sulfuric acid electrolysis, the passive film existing on the surface of stainless steel was effectively removed, elution of the base material occurred, and a surface with higher Sa and Sdr was formed. A high Sdr means that, unlike the apparent area due to a rough surface shape rather than a flat surface, the developed area increased significantly. Therefore, when using stainless steel with a high Sdr as a separator, it is expected that the actual contact area with the GDL in the stack will increase significantly, and a low contact resistance value will be ensured.

[0044] Looking at the surface shape control process in Table 2, it can be seen that it is possible to secure a low contact resistance of 10 mΩ·cm or less even by simply immersing in a hydrochloric acid solution, but it is more preferable to add the step of immersing in a mixed acid solution. Also, the sulfuric acid electrolysis process is performed twice, first and second. After the first electrolysis treatment at a current density of 0.15 to 0.45 A / cm 2 2 and then at a current density of 0.03 to 0.07 A / cm 2 2 ​It can be seen that it is preferable to perform the secondary electrolysis treatment at the current density of . However, not only the surface shape control process in Table 2, but also at least one surface can be controlled with Sa of 0.05 μm or more and Sdr of 5% or more through various processes such as electrolysis and immersion using an acid solution, and a low contact resistance of 10 mΩ·cm 2 can be ensured. Further, it may be replaced with an inorganic acid solution including hydrochloric acid, nitric acid, sulfuric acid, acetic acid, etc. which may cause stainless steel surface dissolution, and a solution containing an oxidizing agent.

[0045] On the other hand, stainless steel containing a Cr content of 15% or less is judged to be difficult to apply as a separator material due to poor corrosion resistance through the surface shape control process.

[0046] FIG. 1 is the 3D surface shape of Comparative Example 3, and FIG. 2 is the 3D surface shape of Example 10. Comparing FIGS. 1 and 2, Comparative Example 3 has a surface without performing the surface shape adjustment process, while Example 10 has fine and sharp irregularities with Sa of 0.111 and Sdr of 29.7 on the surface by undergoing the surface shape adjustment process, and it can be seen that the developed area of the separator surface can be greatly increased. A separator having a surface shape as shown in FIG. 2 ensures a wide actual contact area with the GDL during actual fuel cell operation and exhibits a low contact resistance, and can improve the performance of the fuel cell.

Industrial Applicability

[0047] According to the present invention, since a separator having a low contact resistance can be manufactured without an expensive coating process in a fuel cell environment, industrial applicability is recognized.

Claims

1. By weight, C: more than 0 and not more than 0.02%, N: more than 0 and not more than 0.02%, Si: more than 0 and not more than 0.4%, Mn: more than 0 and not more than 0.3%, P: more than 0 and not more than 0.04%, S: more than 0 and not more than 0.02%, Cr: 15 - 34%, Cu: more than 0 and not more than 1%, Ni: more than 0 and less than 0.4%, containing at least one of Ti and Nb, the composition ratio of the contained elements is more than 0 and not more than 0.5%, the balance being Fe and other inevitable impurities, The stainless steel for a fuel cell separator plate is characterized in that at least one surface has a three-dimensional arithmetic mean roughness (Sa) of 0.05 μm or more and a surface area increase ratio (developed interfacial area ratio, Sdr) of 5% or more.

2. The contact resistance of the stainless steel is 10 mΩ·cm ２ The stainless steel for a fuel cell separator plate according to claim 1, characterized in that it is as follows

3. By weight, C: more than 0 and not more than 0.02%, N: more than 0 and not more than 0.02%, Si: more than 0 and not more than 0.4%, Mn: more than 0 and not more than 0.3%, P: more than 0 and not more than 0.04%, S: more than 0 and not more than 0.02%, Cr: 15 - 34%, Cu: more than 0 and not more than 1%, Ni: more than 0 and less than 0.4%, containing at least one of Ti and Nb, the composition ratio of the contained elements is more than 0 and not more than 0.5%, the balance being Fe and other inevitable impurities, a step of manufacturing a cold-rolled thin sheet by hot rolling and cold rolling a stainless steel having at least one surface with a three-dimensional arithmetic mean roughness (Sa) of 0.05 μm or more and a surface area increase ratio (developed interfacial area ratio, Sdr) of 5% or more; immersing the cold-rolled thin sheet in an acid solution; performing a primary electrolysis treatment at a current density of 0.15 - 0.45 A / cm2 and then performing a secondary electrolysis treatment at a current density of 0.03 - 0.07 A / cm2; further comprising a step of immersing in a mixed acid solution, a method for manufacturing a stainless steel for a fuel cell separator plate.

4. The method for manufacturing a stainless steel for a fuel cell separator plate according to claim 3, wherein the acid solution contains hydrochloric acid or sulfuric acid.

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