Manufacturing method for fuel cell separators

Abstract

To provide a manufacturing method of a fuel cell separator with further improved corrosion resistance.SOLUTION: A manufacturing method of a stainless steel fuel cell separator includes performing laser treatment on a contact surface with liquid water of a stainless steel substrate formed into the shape of a separator to modify a passive film on the surface of the stainless steel substrate to the passive film rich in Cr and / or Mn so as to reduce the relative amount of Fe in the passive film.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing a separator for fuel cells. [Background technology]

[0002] Solid polymer fuel cells (hereinafter referred to as fuel cells), which use electrolyte membranes, can operate at low temperatures and can be made small and lightweight, so their application to automobiles and other vehicles is being considered.

[0003] In such fuel cells, multiple cells (single cells), which are the basic units, are stacked together. Each cell comprises a membrane electrode assembly with a pair of gas diffusion layers (GDLs) on both sides, and a pair of separators that sandwich them. The membrane electrode assembly has a structure in which an anode electrode and a cathode electrode are positioned on both sides of an electrolyte membrane, which is an ion exchange membrane.

[0004] Stainless steel separators, which offer excellent corrosion resistance and are inexpensive, are widely used as separators for fuel cells. While stainless steel separators generally achieve corrosion resistance through a passive film of chromium oxide (typically Cr2O3), the actual components of this passive film include oxides and hydroxides such as FeO and FeOH. Furthermore, exposure of the stainless steel surface to liquid water, such as cooling water used during fuel cell operation or water generated during operation, can cause corrosion. Therefore, when using stainless steel substrates for fuel cell separators, there are areas where further improvement in corrosion resistance is necessary. [Overview of the project] [Problems that the invention aims to solve]

[0005] As mentioned above, in the manufacture of conventional stainless steel fuel cell separators, there has been a need for improved corrosion resistance. Therefore, the present invention aims to provide a method for manufacturing fuel cell separators with further improved corrosion resistance.

Means for Solving the Problem

[0006] The inventors have found that the corrosion resistance can be further improved by performing laser treatment on the contact surface of a stainless steel substrate with liquid water, and thus completed the present invention.

[0007] That is, the gist of the present invention is as follows. (1) A method for manufacturing a separator for a fuel cell made of stainless steel, including performing laser treatment on the contact surface of a stainless steel substrate formed into the shape of the separator to modify the passive film on the surface of the stainless steel substrate, where the passive film of the laser-treated portion on the surface of the stainless steel substrate is determined by X-ray photoelectron spectroscopy (XPS), the ratio (Cr / Fe ratio) of the Cr content (Atomic%) to the Fe content (Atomic%) is greater than 1 when the value before laser treatment is set to 1, and / or the ratio (Mn / Fe ratio) of the Mn content (Atomic%) to the Fe content (Atomic%) is greater than 1 when the value before laser treatment is set to 1 A method for manufacturing a separator for a fuel cell, which is modified to a passive film. (2) The energy density of the laser is 10 mJ / mm , , , , 2 , , 2 , ,

[0008] , , , [Figure 1] ,

[0009] , 2 , 2 , , , , , ~40 mJ / mm 2 The method for manufacturing a separator for a fuel cell according to (1) above. (3) The energy density of the laser is 10 mJ / mm 2 ~20 mJ / mm 2 The method for manufacturing a separator for a fuel cell according to (1) or (2) above.

Advantages of the Invention

[0008] According to the present invention, it becomes possible to provide a method for manufacturing a separator for a fuel cell with further improved corrosion resistance.

Brief Description of the Drawings

[0009] [Figure 1]Figures 1A to C show the results of XPS analysis in the depth direction of the passive film before the corrosion test in the examples (Figure 1A: 20 mJ / mm2 laser treatment part, Figure 1B: 30 mJ / mm2 laser treatment part, Figure 1C: non-laser-treated part). [Figure 2] Figure 2A shows the XPS spectrum representing the bonding state of Cr in the passive film before the corrosion test in the examples. Figure 2B shows the XPS spectrum representing the bonding state of O in the passive film before the corrosion test in the examples. [Figure 3] Figures 3A to C show the changes in the Cr / Fe ratio for each corrosion test time in the laser-treated and non-treated parts in the examples (Figure 3A: 20 mJ / mm2 laser treatment part, Figure 3B: 30 mJ / mm2 laser treatment part, Figure 3C: non-laser-treated part). [Figure 4] Figures 4A and B show the results of XPS analysis in the depth direction of the passive film of the laser-treated part after the corrosion test in the examples (Figure 4A: 20 mJ / mm2 laser treatment part, Figure 4B: 30 mJ / mm2 laser treatment part). [Figure 5] Figures 5A and B show the results of XPS analysis in the depth direction of the passive film of the non-laser-treated part (corroded part) after the corrosion test in the examples (Figure 5A: non-treated part of the 20 mJ / mm2 laser-treated sample, Figure 5B: non-treated part of the 30 mJ / mm2 laser-treated sample). [Figure 6] Figure 6A shows the XPS spectrum representing the bonding state of Cr in the passive film after the corrosion test in the examples. Figure 6B shows the XPS spectrum representing the bonding state of O in the passive film after the corrosion test in the examples.

Mode for Carrying Out the Invention

[0010] Hereinafter, preferred embodiments of the present invention will be described in detail.

[0011] The present invention relates to a method for manufacturing a stainless steel fuel cell separator from a stainless steel substrate. The method for manufacturing a fuel cell separator according to the present invention includes laser treatment of the contact surface of the stainless steel substrate, which has been molded into the shape of a separator, with the liquid water (generated water or cooling water), particularly around the manifold.

[0012] The stainless steel used as the base material is not particularly limited, but examples include austenitic stainless steels such as SUS304, SUS316, and SUS430 as specified in JIS G 4305:2015, and ferritic stainless steels such as SUS444.

[0013] The thickness of the stainless steel substrate is not particularly limited, but is usually 80 μm to 200 μm, and preferably 80 μm to 100 μm.

[0014] In this invention, a laser is irradiated onto the surface of a stainless steel substrate that has been formed into the shape of a separator to modify the passive film on the surface of the stainless steel substrate. The stainless steel substrate can be formed into the shape of a separator, for example, by press molding. The shape of the separator can be any shape that is typical for a fuel cell separator, for example, a shape in which numerous grooves are formed in an uneven manner along the longitudinal direction, and flow paths for fuel gas, oxidizer gas, and refrigerant are formed, but it is not limited to this shape as long as these flow paths can be secured.

[0015] In the present invention, by irradiating the surface of a stainless steel substrate formed into the shape of a separator with a laser, the corrosion resistance of the site processed by the laser (laser processing section) can be improved. A separator for a fuel cell usually comes into contact with generated water as liquid water on the gas surface side that contacts a fuel gas or an oxidant gas, and contacts cooling water as liquid water on the cooling surface side opposite to the thickness direction. In particular, generated water tends to accumulate around the manifold through which the fuel gas and air flow, and electrochemical corrosion occurs easily in the cooling water manifold using cooling water as an electrolyte. In the present invention, since the corrosion resistance is improved at the site where the surface of the stainless steel substrate is processed by the laser, it is preferable to perform laser processing on the surface around the manifold that contacts the generated water and on the periphery of the cooling water manifold where electrochemical corrosion occurs due to contact with the cooling water. In the present invention, the laser processing may be performed at least on the site that contacts the liquid water around the manifold on the surface of the stainless steel substrate, or may be performed on the entire surface of the stainless steel substrate.

[0016] The laser processing is preferably performed such that the energy density of the laser (hereinafter also referred to as fluence) and the processing speed satisfy the following formula (1). Fluence E (mJ / mm 2 ) / processing speed V (mm / s) = α Formula (1) (In Formula (1), α is preferably 0.0033 to 0.0133, more preferably 0.0033 to 0.0067, and particularly preferably 0.0067.)

[0017] The fluence of the laser is preferably 10 mJ / mm 2 ~40 mJ / mm 2 and more preferably 10 mJ / mm 2 ~20 mJ / mm 2 or more than 20 mJ / mm 2 ~40 mJ / mm 2 and particularly preferably 10 mJ / mm 2 ~l20 mJ / mm 2 is. When the fluence of the laser is 10 mJ / mm 2 ~40 mJ / mm 2This allows for the dissolution of a passive film several nanometers thick on the surface of the stainless steel substrate before laser treatment, forming a new passive film rich in Cr and / or Mn with high corrosion resistance. Furthermore, if the laser fluence is 10 mJ / mm², 2 ~20mJ / mm 2 This allows for the formation of a more Cr and Mn-rich passivation film, further improving corrosion resistance. Furthermore, the low energy extends the lifespan of the laser oscillator, which is advantageous in terms of reducing running costs.

[0018] The laser processing speed should be selected to satisfy equation (1) above, for example, 1000 mm / s to 5000 m / s, preferably 2000 mm / s to 4000 mm / s.

[0019] The manufacturing method of the present invention described above yields a stainless steel fuel cell separator with improved corrosion resistance in the laser-treated areas. The fuel cell separator obtained by the manufacturing method of the present invention has a passive coating on the surface of the stainless steel substrate that has been modified by laser treatment to a passive coating with improved corrosion resistance.

[0020] In the fuel cell separator obtained by the manufacturing method of the present invention, the thickness of the passivation film of chromium oxide (generally Cr2O3) in the laser-treated area on the surface of the stainless steel substrate is preferably 1 nm to 50 nm, and more preferably 1 nm to 25 nm. The thickness of the passivation film in the laser-treated area varies depending on the laser fluence. For example, if the laser fluence is 20 mJ / mm 2 In this case, the thickness of the passivation coating in the laser-treated area is the same as before laser treatment, and is usually around 1 nm to 3 nm. Also, the laser fluence is 30 mJ / mm². 2 In this case, the thickness of the passivation coating in the laser-treated area increases compared to before laser treatment, and is typically around 15 nm to 25 nm.

[0021] In a fuel cell separator obtained by the manufacturing method of the present invention, the passivation film on the laser-treated surface of the stainless steel substrate is modified to a passivation film in which the ratio of Cr content (Atomic%) to Fe content (Atomic%) (Cr / Fe ratio), determined by X-ray photoelectron spectroscopy (XPS), is greater than 1 when the pre-laser treatment is set to 1, and / or the ratio of Mn content (Atomic%) to Fe content (Atomic%) (Mn / Fe ratio), is greater than 1 when the pre-laser treatment is set to 1. In other words, the passivation film on the laser-treated surface of the stainless steel substrate is modified to a passivation film in which at least one of the Cr / Fe ratio and the Mn / Fe ratio is greater than 1 when the pre-laser treatment is set to 1.

[0022] In fuel cell separators obtained by the manufacturing method of the present invention, the Cr / Fe ratio in the passivation film of the laser-treated surface of the stainless steel substrate, as determined by XPS analysis, is typically 0.7 to 3.0, with the ratio before laser treatment set to 1. The Cr / Fe ratio in the passivation film of the laser-treated surface varies depending on the laser fluence. For example, when the laser fluence is 20 mJ / mm 2 In this case, the Cr / Fe ratio in the passivation coating of the laser-treated area increases compared to before laser treatment. That is, it is greater than 1 when the value before laser treatment is set to 1. Also, the laser fluence is 30 mJ / mm². 2 In this case, the Cr / Fe ratio in the passivation coating of the laser-treated area decreases compared to before laser treatment. That is, it is less than 1 when the value before laser treatment is set to 1, but the Mn / Fe ratio is greater than 1.

[0023] In one embodiment, the laser fluence is 20 mJ / mm 2In this case, in the fuel cell separator obtained by the manufacturing method of the present invention, the thickness of the passivation film on the laser-treated surface of the stainless steel substrate is the same as before laser treatment, usually about 1 nm to 3 nm, and the Cr / Fe ratio and Mn / Fe ratio in the passivation film increase compared to before laser treatment (i.e., they are greater than 1 when the value before laser treatment is set to 1). In this embodiment, although the thickness of the passivation film on the laser-treated surface is the same as before laser treatment, the Cr / Fe ratio and Mn / Fe ratio in the passivation film increase significantly. That is, it is presumed that the Fe compound in the film is relatively reduced because the passivation film is composed of more Cr and Mn-rich oxides, and the corrosion resistance of the laser-treated surface is improved.

[0024] In another embodiment, the laser fluence is 30 mJ / mm 2 In this case, in the fuel cell separator obtained by the manufacturing method of the present invention, the thickness of the passivation film on the laser-treated surface of the stainless steel substrate increases compared to before laser treatment, and is usually about 15 nm to 25 nm. Furthermore, the Cr / Fe ratio in the passivation film decreases compared to before laser treatment (i.e., it is less than 1 when the value before laser treatment is set to 1), and the Mn / Fe ratio in the passivation film increases compared to before laser treatment (i.e., it is greater than 1 when the value before laser treatment is set to 1). In this embodiment, although the passivation film is not a Cr-rich oxide, it is presumed that the corrosion resistance of the laser-treated surface is improved due to the further increase in the thickness of the passivation film and the improvement in the Mn / Fe ratio. Thus, in the present invention, improving the Mn / Fe ratio in the passivation film is also important for improving corrosion resistance.

[0025] The fuel cell separator of the present invention can have its corrosion resistance in corrosive environments improved by laser treatment. In the present invention, a corrosive environment refers to conditions such as immersing the separator in simulated generated water (pH=3~5) at a temperature of 70°C to 100°C or lower with a potential of 0.8V to 1.0V for 24 hours (hr) or more.

[0026] In the fuel cell separator of the present invention, the Cr / Fe ratio in the passivation film of the laser-treated surface of the stainless steel substrate, as determined by XPS analysis, increases over time in the aforementioned corrosive environment. On the other hand, in the untreated surface, the Cr / Fe ratio in the passivation film decreases over time. In the manufacturing method of the fuel cell separator of the present invention, by applying laser treatment, the corrosion resistance of the resulting fuel cell separator in a corrosive environment is increased, and corrosion can be prevented from progressing at all even in a corrosive environment. [Examples]

[0027] The present invention will be described in more detail below using examples. However, the technical scope of the present invention is not limited to these examples.

[0028] <Corrosion Test> SUS304 stainless steel substrate was used. A portion of the surface of the SUS304 substrate had an energy density (fluence) of 20 mJ / mm². 2 or 30 mJ / mm 2 Evaluation samples were obtained by performing laser processing at a processing speed of 3000 mm / s.

[0029] For the evaluation samples, a constant potential corrosion test was conducted in a corrosive environment simulating the generated water. Specifically, the evaluation samples were immersed for 60 hours in simulated generated water (pH=3) at a temperature of 70°C to 100°C with a potential of 0.8V to 1.0V. After the corrosion test, the presence or absence of corrosion (discoloration) was visually confirmed.

[0030] In the corrosion test, 20 mJ / mm 2 and 30 mJ / mm 2 In all evaluation samples of the fluence, the laser-treated areas showed no discoloration and no corrosion, while the untreated areas showed discoloration to brown to purple and corrosion was observed. Therefore, it was confirmed that laser treatment improves the corrosion resistance of the SUS304 substrate. The difference in corrosion color (brown to purple) in the untreated areas is thought to be due to variations in the thickness and composition of the passive film on the surface of the SUS304 substrate.

[0031] <Analysis of the mechanism for improving corrosion resistance> To analyze the mechanism of improved corrosion resistance confirmed in corrosion tests, X-ray photoelectron spectroscopy (XPS) analysis was performed on evaluation samples before and after the corrosion tests to analyze the elements in the surface and depth directions of the passivation film on the SUS304 substrate surface. XPS analysis was performed using an XPS instrument (ULVAC; PHI5000 VersaProbeII) under the following conditions: X-ray source; AlKα monochromatic light, output 25W, voltage 15kV, irradiation area; φ100μm, analysis area; 1000×200μm, neutralization gun; ON state, pulse energy (wide; 187.85eV, narrow; 46.95-117.40eV), step size (wide; 0.4eV, narrow; 0.1eV), shift correction was performed with C1s; CC, CH, 284.8eV. Depth analysis was performed while etching with Ar monomer at a voltage of 3kV, irradiation area of ​​2×2mm, and etching speed (sputtering speed) of 6.5nm / min.

[0032] 1. Surface analysis of the passivation film before corrosion testing 1-1. Elemental analysis of the outermost surface of the passive coating before corrosion testing. Table 1 shows the elemental analysis results of the outermost surface of the passive coating before corrosion testing. As shown in Table 1, a low fluence of 20 mJ / mm² was observed. 2 In the laser-treated area, the amounts of Cr and Mn increased, while the amount of Fe decreased compared to the untreated area. On the other hand, with a high fluence of 30 mJ / mm², 2 In the laser-treated section, the amount of Cr decreased, while the amounts of Fe, Mn, and O increased compared to the untreated section. The Cr / Fe ratio in the laser-treated section was 20 mJ / mm², which is a low fluence. 2 However, this is higher than the laser-untreated area, but with a high fluence of 30 mJ / mm². 2 The Mn / Fe ratio was lower in the laser-treated area than in the untreated area. On the other hand, the Mn / Fe ratio was higher in the laser-treated area than in the untreated area, regardless of the fluence. An improvement in the Mn / Fe ratio indicates that the amount of Fe that combines with oxygen is relatively reduced in the passive coating, and this is also thought to contribute to improved corrosion resistance.

[0033] [Table 1]

[0034] 1-2. Analysis of the passivation film in the depth direction before corrosion testing. Figures 1A-C show the results of XPS analysis in the depth direction of the passivation film before corrosion testing (sputtering rate: 6.5 nm / min) (Figure 1A: 20 mJ / mm²). 2 Laser processing unit, Figure 1B: 30 mJ / mm 2 (Laser processing section, Figure 1C: Unprocessed area). The thickness of the passivation film (≒ oxide film) was determined from Figures 1A-C. Specifically, the thickness of the passivation film is generally defined as half of the maximum oxygen content, so the thickness of the passivation film was calculated by converting the values ​​on the horizontal axis (sputtering time) of the graph based on this (indicated by the broken line arrow in Figures 1A-C). As a result, the laser fluence was 20 mJ / mm. 2 The thickness of the passive film is approximately 2.3 nm (Figure 1A), and the laser fluence is 30 mJ / mm². 2 The passivation coating thickness was approximately 19.5 nm, which is considerably thicker (Figure 1B), while the passivation coating thickness of the untreated area was approximately 2.0 nm (Figure 1C), which is consistent with the generally accepted thickness of the passivation coating on stainless steel (1-2 nm).

[0035] Based on these factors, a low fluence of 20 mJ / mm² is considered. 2 In the laser-treated area, corrosion resistance was found to be improved despite the passivation film thickness being almost the same as that of the untreated area. As can be seen from the XPS spectrum representing the Cr bonding state in the passivation film before corrosion testing shown in Figure 2A, and the XPS spectrum representing the O bonding state in the passivation film before corrosion testing shown in Figure 2B, the corrosion resistance was 20 mJ / mm². 2 In the laser-treated area, the Cr and O spectra are sharper in the Cr2O3 range compared to the untreated area, suggesting that the passivation film is composed of a more Cr-rich oxide, thus improving corrosion resistance. 30 mJ / mm 2 In the laser processing area, the amount of Cr in the passivation film is low, but it is 20 mJ / mm 2Similar to the laser processing unit, the sharpness of the Cr and O spectra in the Cr2O3 range, and the thickness of the passivation film, are presumed to contribute to the improved corrosion resistance. Thus, it is considered that the sharp shape of the Cr and O intensities in the Cr2O3 region in the XPS spectrum, which represents the bonding state of Cr and O in the passivation film, is important for improving corrosion resistance.

[0036] 2. Surface analysis of the passivation film after corrosion testing 2-1. Elemental analysis of the outermost surface of the passive coating after corrosion testing Table 2 shows the elemental analysis results of the outermost surface of the passive coating after the corrosion test. As shown in Table 2, regardless of the laser fluence, the laser-treated area had less Fe and more Cr compared to the untreated (corroded) area. Figures 3A-3C show the change in the Cr / Fe ratio at each corrosion test time for the laser-treated and untreated areas (Figure 3A: 20 mJ / mm²). 2 Laser processing unit, Figure 3B: 30 mJ / mm 2 Figure 3C shows the laser-treated surface (untreated surface). From Figures 3A and 3B, the Cr / Fe ratio increased with the corrosion test time in the laser-treated surface. This means that in the laser-treated surface, the corrosion resistance strength increases in corrosive environments such as contact with generated water (i.e., corrosion does not progress at all). On the other hand, from Figure 3C, in the untreated surface (SUS304 substrate), the Cr / Fe ratio decreased with the corrosion test time compared to the initial (before corrosion test, 0 hr) Cr / Fe ratio of the passive coating surface, and eventually tended to become zero. This means that the corrosion resistance deteriorates if the stainless steel substrate is left as is without modification of the passive coating by laser treatment.

[0037] [Table 2]

[0038] 2-2. Analysis of the passivation film in the depth direction after corrosion testing. The results described in 2-1 above were confirmed by XPS analysis of the passivation film in the depth direction. Figures 4A and 4B show the results of XPS analysis of the passivation film in the depth direction of the laser-treated section after the corrosion test (sputtering rate: 6.5 nm / min) (Figure 4A: 20 mJ / mm²). 2 Laser processing unit, Figure 4B: 30 mJ / mm 2 (Laser-treated area). Figures 5A and 5B also show the results of XPS analysis (sputtering rate: 6.5 nm / min) of the depth direction of the passivation coating in the untreated area (corroded area) after the corrosion test (Figure 5A: 20 mJ / mm²). 2 Untreated laser-treated sample, Figure 5B: 30 mJ / mm 2 (Untreated laser-treated sample). From Figures 4A and 4B, the Cr-rich layer increased and the thickness of the passive film increased in the laser-treated sample (indicated by the broken line arrows (I) and (II) in Figures 4A and 4B), whereas from Figures 5A and 5B, the untreated sample had changed into an oxide film with almost no Cr. As can be seen from the XPS spectrum representing the Cr bonding state in the passive film after the corrosion test shown in Figure 6A, and the XPS spectrum representing the O bonding state in the passive film after the corrosion test shown in Figure 6B, the laser-treated sample had a Cr2O3 passive film, while the untreated sample had changed into an oxide film mainly composed of Fe2O3.

Claims

1. A method for manufacturing a stainless steel fuel cell separator, comprising applying laser treatment to the contact surface with liquid water of a stainless steel substrate formed into the shape of a separator, thereby modifying the passivation film on the surface of the stainless steel substrate. The passivation film on the surface of the stainless steel substrate treated with the laser is determined by X-ray photoelectron spectroscopy (XPS). The ratio of Cr content (atomic%) to Fe content (atomic%) (Cr / Fe ratio) is greater than 1 when the value before laser treatment is set to 1, and / or The ratio of Mn content (atomic%) to Fe content (atomic%) (Mn / Fe ratio) is greater than 1 when the value before laser treatment is set to 1. A method for manufacturing a fuel cell separator that is modified into a passivation film.

2. The energy density of the laser is 10 mJ / mm². ２ ~40 mJ / mm ２ The method for manufacturing a fuel cell separator according to claim 1.

3. The energy density of the laser is 10 mJ / mm². ２ ~20 mJ / mm ２ The method for manufacturing a fuel cell separator according to claim 1 or 2.

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