Stainless steel for fuel cell separator plates and its manufacturing method

Abstract

We provide stainless steel with low contact resistance by forming minute protrusions on the surface as a material for fuel cell separator plates. The present invention relates to a metal material for a fuel cell separator, the surface of which contacts with a GDL in a fuel cell has 5 or more micro-projections with a size of 10-100 nm, and the ratio of real surface length to apparent surface length without projections is 1.15 or more based on a cross section observed with a transmission electron microscope. The metal material forms micro-projections on the surface, and the contact resistance is 10 mΩ cm. 2 The present invention is characterized in that:

Description

[Technical Field]

[0001] The present invention relates to a stainless steel for a separator plate of a fuel cell and a manufacturing method thereof, and more particularly to a stainless steel for a separator plate of a fuel cell that can control the surface shape of the separator plate to increase the contact area with a gas diffusion layer (GDL) and ensure low contact resistance. [Background technology]

[0002] Generally, a fuel cell stack is a stack of cells, each consisting of a membrane electrode assembly (MEA) containing an electrolyte, electrodes, and a GDL, and a separator. The separator is in contact with the GDL, and contact resistance at the separator / GDL interface reduces the performance of the cell and fuel cell.

[0003] The contact resistance of such separators is primarily influenced by two factors. First, there is the passivation film, an oxide layer on the surface of the metal separator. Passivation films are a method of ensuring high corrosion resistance, but because they are non-conductive oxide layers, it is preferable to have as thin a thickness as possible. Second, the contact resistance is affected by the contact area between the separator and GDL. Because the separator and GDL each have different surface roughness, the actual contact area between the two objects in contact has a significant impact on contact resistance. A large contact area between the separator and GDL tends to result in low contact resistance, while a small contact area tends to result in high contact resistance. Therefore, the effect of reducing contact resistance varies depending on how the surface shape of the separator is modified.

[0004] Generally, a fuel cell stack is a stack of cells, each consisting of a membrane electrode assembly (MEA) containing an electrolyte, electrodes, and a GDL, and a separator. The separator is in contact with the GDL, and the resistance at the separator / GDL interface is called contact resistance, which determines the performance of the cell and the fuel cell.

[0005] The contact resistance of such separators is primarily influenced by two factors. The first is the passivation film, an oxide layer that forms on the surface of the metal separator. While a passivation film can ensure high corrosion resistance, it is a non-conductive oxide layer, so it is preferable to have as thin a thickness as possible. The second factor that affects contact resistance is the contact area between the separator and GDL. Because the separator and GDL each have different surface roughness, the actual contact area between the two objects in contact has a significant impact on contact resistance. A large contact area between the separator and GDL results in low contact resistance, while a small contact area results in high contact resistance. Therefore, the effectiveness of reducing contact resistance varies depending on the surface shape of the separator.

[0006] The separator plate material is an extremely thin material with a thickness of about tens to hundreds of micrometers, and this ultra-thin material is subjected to bright annealing during the manufacturing process. This results in a bright annealed surface with almost no surface irregularities. If such a material is used as a separator plate as is, the contact area with the GDL will be small, resulting in high contact resistance. Therefore, to reduce contact resistance, it is necessary to make the separator plate surface irregular to increase the contact area with the GDL. In this case, it is advantageous for the surface to have fine protrusions several tens of nanometers high. Summary of the Invention [Problem to be solved by the invention]

[0007] SUMMARY OF THE INVENTION In view of the limitations and problems of the prior art, the object of the present invention is to provide a stainless steel material for fuel cell separator plates that has fine protrusions formed on its surface and has low contact resistance. [Means for solving the problem]

[0008] The metal material for fuel cell separators of the present invention is characterized in that the surface of the metal material that comes into contact with the GDL in the fuel cell has five or more fine protrusions of 10 to 100 nm in size, and the ratio of the real surface length to the apparent surface length without protrusions is 1.15 or more, based on a cross section observed with a transmission electron microscope (TEM).

[0009] The metal material for fuel cell separator plates of the present invention is preferably a ferritic stainless steel consisting of, by weight, 15 to 35% Cr, 0.02% or less C, 0.02% or less N, 0.4% or less Si, 0.003% or less S, 0.2% or less Mn, 2% or less Cu, with the remainder being Fe and other unavoidable impurities.

[0010] Another metal material for fuel cell separator plates of the present invention is preferably an austenitic stainless steel consisting of, by weight, 15 to 30% Cr, 7 to 15% Ni, 0.09% or less C, 2.5% or less Si, 0.003% or less S, 3% or less Mn, 3% or less Mo, 0.3% or less N, with the remainder being Fe and other unavoidable impurities. [Effects of the Invention]

[0011] According to the present invention, the separator plate manufacturing method of the present invention allows for the manufacture of separator plates with low contact resistance in a fuel cell environment without expensive coating processes. [Brief explanation of the drawings]

[0012] [Figure 1] This is a transmission electron microscope image of a cross section of a fuel cell separator plate according to the present invention, which has five or more fine protrusions with a height of 10 to 100 nm or less and whose actual surface length is 15% or more longer than the apparent surface length without the protrusions. [Figure 2] FIG. 10 is a cross-section of a fuel cell separator plate without protrusions, observed with a transmission electron microscope. DETAILED DESCRIPTION OF THE INVENTION

[0013] In one embodiment of the present invention, the metal material for a fuel cell separator has a surface that comes into contact with the GDL in the fuel cell and has five or more micro-protrusions of 10 to 100 nm in size, and the ratio of the real surface length to the apparent surface length without the protrusions is 1.15 or more based on a cross section observed with a transmission electron microscope (TEM).

[0014] The surface of bright annealed stainless steel sheets has macroscopic rolling marks that occur in the rolling direction during cold rolling. These rolling marks have a roughness that is too large in scale to increase the contact area between the separator plate material and the GDL, so they do not have a significant effect on the contact resistance. Furthermore, because they are not distributed uniformly on the surface, they do not have a significant effect on the contact area of ​​the entire surface.

[0015] Therefore, by uniformly distributing fine protrusions of 100 nm or less on the surface of stainless steel, the actual surface area is increased compared to the apparent surface area, thereby increasing the contact area with the GDL and reducing contact resistance. If the protrusion height is too low, less than 10 nm, the effect on increasing the surface area is minimal. If the protrusion height is too high, more than 100 nm, the number of protrusions per unit area is reduced, resulting in a reduced effect on increasing the surface area. When fine protrusions with a height of 10 to 100 nm are uniformly formed on the surface, the actual surface length can be increased by more than 15% compared to the apparent surface length without the protrusions, thereby increasing the contact area with the GDL. If the actual surface length is increased by more than 15% in one direction of the surface, the actual contact area can increase by more than 30%, resulting in a reduction in contact resistance. In the present invention, fine protrusions with a height of 10 to 100 nm refer to the presence of a large number of protrusions per unit area that are effective in increasing the contact area with the GDL. Therefore, the separator according to the present invention has a contact resistance of 10 mΩ·cm. 2 The present invention is characterized by the following:

[0016] Hereinafter, unless otherwise specified, the unit is % by weight. Furthermore, when a certain 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.

[0017] [Ferritic stainless steel] In one embodiment of the present invention, the metal material for a fuel cell separator is preferably a ferritic stainless steel consisting of, by weight, 15 to 35% Cr, 0.02% or less C, 0.02% or less N, 0.4% or less Si, 0.003% or less S, 0.2% or less Mn, 2% or less Cu, with the remainder being Fe and other unavoidable impurities. According to another embodiment of the present invention, the metal material for a fuel cell separator preferably further contains at least one element selected from the group consisting of Ti, Nb, and V in a total amount of 1.0% or less of the ferritic stainless steel.

[0018] C: 0.02% or less It is an austenite-forming element that improves high-temperature strength when added, but when added in excess, it reacts with Cr to form chromium carbide, reducing corrosion resistance and also reducing elongation and weldability in ferritic steels. Therefore, it is preferable to keep the content as low as possible, 0.02% or less.

[0019] N: 0.02% or less It is an element that stabilizes the austenite phase and has the advantage of improving strength and pitting corrosion resistance as a substitute for Ni, but has the disadvantage of reducing workability such as elongation. Therefore, in the present invention, it is limited to 0.02% or less.

[0020] Si: 0.4% or less Although it has the advantage of improving high-temperature oxidation resistance and strengthening the passive film in stainless steel to improve corrosion resistance, excessive addition reduces the elongation rate, so it is limited to 0.4% or less.

[0021] S: 0.003% or less It is a trace impurity element that segregates at grain boundaries and is the main element that causes processing cracks during hot rolling, so its content is limited to as low as possible, 0.003% or less.

[0022] Mn: 0.2% or less Like nitrogen, it is an austenite phase stabilizing element. When added to ferritic steel as a substitute for Ni to metastabilize the austenite phase, it increases strength and reduces workability, so it is limited to 0.2% or less.

[0023] Cu: 2% or less It is an element that stabilizes the austenite phase and is useful for improving corrosion resistance, but excessive addition can reduce hot workability, so it should be limited to 2% or less.

[0024] Cr: 15~35% Cr is an element that promotes the formation of oxides in stainless steel. For corrosion resistance, 15% or more Cr must be added. However, if added in excess, there is a problem of increased sticking defects due to the formation of dense oxide scale during hot rolling, so the Cr content is limited to 35%.

[0025] Ti, Nb, V: 1.0% or less in total It is an element that is effective in forming carbonitrides of C and N in steel, but it also reduces toughness, so the sum of the respective components is limited to 1.0% or less.

[0026] [Austenitic stainless steel] In one embodiment of the present invention, the metal material for a fuel cell separator is preferably an austenitic stainless steel containing, by weight, 15-30% Cr, 7-15% Ni, 0.09% or less C, 2.5% or less Si, 0.003% or less S, 3% or less Mn, 3% or less Mo, 0.3% or less N, with the remainder being Fe and other unavoidable impurities. According to another embodiment of the present invention, the metal material for a fuel cell separator preferably further contains at least one element selected from the group consisting of Ti, Nb, and V in a total amount of 1.0% or less.

[0027] C: 0.09% or less It is an element that stabilizes the austenite phase and is necessary for austenitic stainless steels. When added, it improves high-temperature strength. However, when added in excess, it reacts with Cr to form chromium carbide, reducing corrosion resistance, and also reducing elongation and weldability in ferritic steels. Therefore, it is preferable to keep the content as low as possible, at 0.09% or less.

[0028] N: 0.3% or less It is an element that stabilizes the austenite phase and has the advantage of improving strength and pitting corrosion resistance as a substitute for Ni, but has the disadvantage of reducing workability such as elongation. Therefore, in the present invention, it is limited to 0.3% or less.

[0029] Si:2.5% or less An element that improves the corrosion resistance of stainless steel. If contained in excess of 2.5%, it reduces the elongation rate and forms SiO2 oxide inclusions, which reduces corrosion resistance, so it is limited to 2.5% or less.

[0030] S: 0.003% or less It is a trace impurity element that segregates at grain boundaries and is the main element that causes processing cracks during hot rolling, so its content is limited to as low as possible, 0.003% or less.

[0031] Mn: 3% or less Like nitrogen, it is an austenite phase stabilizing element and can replace Ni, but excessive addition reduces corrosion resistance, so it is limited to 3.0% or less.

[0032] Mo: 3% or less It is an effective element for improving the corrosion resistance of stainless steel, but if added in excess, it can cause corrosion resistance and brittleness due to the formation of sigma phases. Since it is an expensive element, it is limited to 3% or less.

[0033] Cr: 15~30% It is an element that promotes the formation of oxides in stainless steel and improves corrosion resistance. To ensure corrosion resistance in a fuel cell environment, it must be at least 15%. If added in excess, it becomes necessary to add more Ni, which is expensive for the stability of the austenite phase, Mn, which reduces corrosion resistance, and N, which reduces workability, so it is limited to 15% to 30%.

[0034] Ni: 7-15% It is an austenite phase stabilizing element and is an expensive element, so its content is limited to 7-15% for economic reasons.

[0035] Ti, Nb, V: 1.0% or less in total It is an element that is effective in forming carbonitrides of C and N in steel, but it also reduces toughness, so the sum of the respective components is limited to 1.0% or less.

[0036] (manufacturing process) The method for adjusting the surface shape of stainless steel may be carried out through the following steps. The surface shape of the stainless steel may be adjusted by a chemical or mechanical method. Using a chemical method, the surface shape can be adjusted by immersing the stainless steel in an acid solution. The acid solution in which the stainless steel is immersed can be hydrochloric acid, sulfuric acid, nitric acid, or hydrofluoric acid. Two or more of these acid solutions can be mixed together, or the stainless steel can be immersed in two or more acid solutions sequentially. Furthermore, the surface shape of the stainless steel may be adjusted by electrolytic treatment, which may include carrying out electrolytic treatment before or after immersion in an acid solution.

[0037] When using the chemical method, the surface shape may vary depending on the type, temperature, concentration, etc. of the acid solution, the immersion time, and the applied current for the electrolytic treatment. For example, in an acid solution, 5% to 20% sulfuric acid at 40 to 60°C is used, and the current is 0.1 to 0.5 A / cm 2 Alternatively, the wire is immersed in a mixed acid (nitric acid and hydrofluoric acid) at 40°C to 60°C for 30 to 300 seconds, resulting in a surface shape with excellent contact resistance.

[0038] Furthermore, for example, a surface shape with excellent contact resistance can be obtained by immersing the substrate in an acid solution of 5 to 20% hydrochloric acid or 5 to 20% hydrofluoric acid for 30 to 300 seconds. The surface shape of the stainless steel may also be adjusted by mechanical polishing instead of the chemical method, and the surface shape will vary depending on the type, thickness, shape and distribution of the abrasive used during polishing. However, the surface shape according to this specification is not limited to the above-mentioned chemical or mechanical methods, and can be derived by various conditions and methods.

[0039] The present invention will be described in more detail below with reference to examples. The following examples are presented to fully convey the concept of the present invention to those skilled in the art, and the present invention is not limited to the examples presented herein and may be embodied in other forms.

[0040] (Example) Table 1 shows the alloy compositions of all examples, including comparative examples and inventive examples, which are ferritic and austenitic stainless steels. The stainless steel used in the present invention was prepared by cold-rolling stainless steel having the above composition using a Z-mill cold rolling mill in the cold-rolling step, and then bright annealing the cold-rolled sheet in the heat treatment step.

[0041] [Table 1]

[0042] Steel A is a ferritic stainless steel according to the present invention, and Steel B is an austenitic stainless steel according to the present invention. In the comparative examples and examples of the present invention, the surface morphologies of the ferritic and austenitic stainless steels were adjusted to investigate the relationship between contact resistance and surface shape index, and the results are shown in Table 2 below.

[0043] [Table 2]

[0044] Specifically, Table 2 shows the surface analysis results and contact resistance measurements of the manufactured cold-rolled steel sheets. The average height of the protrusions, the number of protrusions between 10 and 100 nm, and the actual surface length / apparent surface length of the steel sheet cross section were measured using a transmission electron microscope (TEM). Surface protrusions that are effective in determining contact resistance are minute protrusions with a height of several tens of nm, making them difficult to observe with a low-magnification scanning electron microscope (SEM). Therefore, the values ​​in Table 2 were measured using a high-magnification transmission electron microscope of 100,000x or more, and the average values ​​were obtained by observing 10 locations per specimen. The interfacial contact resistance in Table 2 was evaluated by preparing two sheets of the manufactured material, placing a carbon paper (SGL-10BA) used as a gas diffusion layer between them, and applying a contact pressure of 100 N / cm. 2 The interfacial contact resistance at each point was evaluated five times, and the average value was calculated.

[0045] Referring to Table 2, Examples 1 to 7 having surface microprotrusions as defined by the present invention have a contact resistance of 10 mΩ·cm 2 The results were excellent below 100 nm. The average height of the protrusions does not have a strong correlation with contact resistance, but the greater the number of fine protrusions between 10 and 100 nm, the lower the contact resistance tended to be. There was also a tendency for the actual surface length to be longer compared to the apparent surface length as the number of fine protrusions between 10 and 100 nm on the surface increased. This is thought to have increased the actual contact area with the GDL as a separator, thereby reducing contact resistance.

[0046] However, in Comparative Examples 1 to 9, the contact resistance was 10 mΩ cm on a surface having no or a very small number of micro-protrusions between 10 and 100 nm, which is the limit of the present invention. 2 exceeded. [Industrial Applicability]

[0047] The method for manufacturing a separator plate according to the present invention can manufacture a separator plate having low contact resistance in a fuel cell environment without an expensive coating process, and therefore has industrial applicability.

Claims

1. The surface of the metal material made of stainless steel, which is in contact with the GDL in the fuel cell, has a plurality of protrusions with an average height of 130 to 150 nm, of which the number of fine protrusions with a height of 10 to 100 nm is 5 or more, and the ratio of the real surface length to the apparent surface length without the protrusions is 1.15 or more based on a cross section observed with a transmission electron microscope (TEM), and the contact resistance is 10 mΩ cm ２ A metal material for a fuel cell separator plate, characterized in that:

2. 2. The metal material for fuel cell separator plates according to claim 1, characterized in that the metal material is a ferritic stainless steel consisting of, by weight percent, 15 to 35% Cr, 0.02% or less C, 0.02% or less N, 0.4% or less Si, 0.003% or less S, 0.2% or less Mn, 2% or less Cu, with the balance being Fe and other unavoidable impurities.

3. 3. The metal material for a fuel cell separator plate according to claim 2, wherein the ferritic stainless steel further contains at least one element selected from the group consisting of Ti, Nb, and V in a total amount of 1.0% or less.

4. 2. The metal material for fuel cell separator plates according to claim 1, characterized in that the metal material is an austenitic stainless steel consisting of, by weight percent, 15 to 30% Cr, 7 to 15% Ni, 0.09% or less C, 2.5% or less Si, 0.003% or less S, 3% or less Mn, 3% or less Mo, 0.3% or less N, with the balance being Fe and other unavoidable impurities.

5. 5. The metal material for a fuel cell separator plate according to claim 4, wherein the austenitic stainless steel further contains at least one element selected from the group consisting of Ti, Nb, and V in a total amount of 1.0% or less.

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