JP7831391B2 - Manufacturing method for fuel cell separators - Google Patents

Description

This invention relates to a method for manufacturing a separator for fuel cells.

Solid polymer fuel cells (hereinafter referred to as "fuel cells"), which use electrolyte membranes, can operate at low temperatures and can be made smaller and lighter, making them suitable for use in automobiles and other applications.

In such fuel cells, multiple cells (single cells), which are the basic units, are stacked. Each cell comprises a membrane electrode assembly with a pair of gas diffusion layers (GDLs) on both sides, and a pair of separators sandwiching these. 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.

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.

As mentioned above, in the manufacturing 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.

The inventors of this invention have discovered that the corrosion resistance of a stainless steel substrate can be further improved by applying laser treatment to the contact surface with liquid water, and have completed this invention.

In other words, the gist of this invention is as follows:
(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).
A method for manufacturing a fuel cell separator, wherein the separator is modified into a passivation film in which the ratio of Cr content (atomic%) to Fe content (atomic%) (Cr/Fe ratio) 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.
(2) The method for manufacturing a fuel cell separator according to (1), wherein the energy density of the laser is 10 mJ/ ^mm² to 40 mJ/ ^mm² .
(3) The method for manufacturing a fuel cell separator according to (1) or (2), wherein the energy density of the laser is 10 mJ/ ^mm² to 20 mJ/ ^mm² .

This invention makes it possible to provide a method for manufacturing a fuel cell separator with further improved corrosion resistance.

Figures 1A to 1C show the results of XPS analysis in the depth direction of the passivation coating before corrosion testing in the examples (Figure 1A: 20 mJ/ ^mm² laser treatment, Figure 1B: 30 mJ/ ^mm² laser treatment, Figure 1C: untreated area). Figure 2A shows the XPS spectrum representing the Cr bonding state in the passivated coating before corrosion testing in the example. Figure 2B shows the XPS spectrum representing the O bonding state in the passivated coating before corrosion testing in the example. Figures 3A to 3C show the change in the Cr/Fe ratio at each corrosion test time for the laser-treated and untreated areas in the example (Figure 3A: 20 mJ/ ^mm² laser-treated area, Figure 3B: 30 mJ/ ^mm² laser-treated area, Figure 3C: untreated area). Figures 4A and 4B show the results of XPS analysis in the depth direction of the passivation coating of the laser-treated section after corrosion testing in the examples (Figure 4A: 20 mJ/ ^mm² laser-treated section, Figure 4B: 30 mJ/ ^mm² laser-treated section). Figures 5A and 5B show the results of XPS analysis in the depth direction of the passivation coating in the untreated (corroded) area after the corrosion test in the example (Figure 5A: untreated area of the 20 mJ/ ^mm² laser-treated sample, Figure 5B: untreated area of the 30 mJ/ ^mm² laser-treated sample). Figure 6A shows the XPS spectrum representing the Cr bonding state in the passivated film after the corrosion test in the example. Figure 6B shows the XPS spectrum representing the O bonding state in the passivated film after the corrosion test in the example.

The following describes preferred embodiments of the present invention in detail.

This invention relates to a method for manufacturing a stainless steel fuel cell separator from a stainless steel substrate. The manufacturing method of the fuel cell separator according to this 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.

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 defined in JIS G 4305:2015, and ferritic stainless steels such as SUS444.

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

In this invention, the passive coating on the surface of a stainless steel substrate, which has been molded into the shape of a separator, is modified by irradiating it with a laser. The stainless steel substrate can be molded into the separator shape, for example, by press molding. The separator shape can be any shape typical of a fuel cell separator; for example, a shape in which numerous grooves are formed along the longitudinal direction, creating passages for fuel gas, oxidizer gas, and refrigerant. However, it is not limited to this shape as long as these passages can be secured.

In this invention, the corrosion resistance of the laser-treated portion (laser-treated area) can be improved by irradiating the surface of a stainless steel substrate molded into the shape of a separator with a laser. Fuel cell separators typically come into contact with generated water as liquid water on the gas side, which is in contact with fuel gas and oxidizer gas, and with cooling water as liquid water on the cooling side, which is opposite the gas side in the thickness direction. In particular, generated water tends to accumulate around the manifold through which fuel gas and air flow, and electrochemical corrosion is likely to occur in the cooling water manifold, where the cooling water acts as the electrolyte. In this invention, since the corrosion resistance is improved in the laser-treated portion of the stainless steel substrate surface, it is preferable to apply laser treatment to the surface around the manifold that comes into contact with generated water, and to the area around the cooling water manifold where electrochemical corrosion occurs due to contact with cooling water. In this invention, laser treatment only needs to be performed on the portion of the stainless steel substrate surface that comes into contact with liquid water around the manifold, but it may also be performed on the entire surface of the stainless steel substrate.

Laser processing is preferably performed such that the laser energy density (hereinafter also referred to as fluence) and processing speed satisfy the following equation (1).
Fluence E (mJ/ ^mm² ) / Processing speed V (mm/s) = α Equation (1)
(In formula (1), α is preferably 0.0033 to 0.0133, more preferably 0.0033 to 0.0067, and particularly preferably 0.0067.)

The laser fluence is preferably 10 mJ/ ^mm² to 40 mJ/ ^mm² , more preferably 10 mJ/ ^mm² to 20 mJ/ ^mm² or greater than 20 mJ/ ^mm² to 40 mJ/ ^mm² , and particularly preferably 10 mJ/ ^mm² to 20 mJ/ ^mm² . When the laser fluence is 10 mJ/ ^mm² to 40 mJ/ ^mm² , the passive film of several nanometers thickness on the surface of the stainless steel substrate before laser treatment can be dissolved, and a new passive film rich in Cr and/or Mn with high corrosion resistance can be formed. Furthermore, when the laser fluence is 10 mJ/ ^mm² to 20 mJ/ ^mm² , an even more Cr and Mn-rich passive film can be formed, further improving corrosion resistance. In addition, the low energy extends the life of the laser oscillator, which is also preferable in terms of reducing running costs.

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.

The manufacturing method of the present invention provides 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.

In a 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, when the laser fluence is 20 mJ/ ^mm² , the thickness of the passivation film in the laser-treated area is the same as before laser treatment, and is usually about 1 nm to 3 nm. When the laser fluence is 30 mJ/ ^mm² , the thickness of the passivation film in the laser-treated area increases compared to before laser treatment, and is usually about 15 nm to 25 nm.

In the fuel cell separator obtained by the manufacturing method of the present invention, the passivation coating on the laser-treated surface of the stainless steel substrate is modified to a passivation coating 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 ratio 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 ratio is set to 1. In other words, the passivation coating on the laser-treated surface of the stainless steel substrate is modified to a passivation coating in which at least one of the Cr/Fe ratio and the Mn/Fe ratio is greater than 1 when the pre-laser treatment ratio is set to 1.

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 value before laser treatment set to 1. The Cr/Fe ratio in the passivation film of the laser-treated surface changes depending on the laser fluence. For example, when the laser fluence is 20 mJ/ ^mm² , the Cr/Fe ratio in the passivation film of the laser-treated surface increases compared to before laser treatment. That is, it is greater than 1 when the value before laser treatment is set to 1. Also, when the laser fluence is 30 mJ/ ^mm² , the Cr/Fe ratio in the passivation film of the laser-treated surface decreases compared to before laser treatment. That is, it is less than 1 when the value before laser treatment is set to 1, but Mn/Fe is greater than 1.

In one embodiment, when the laser fluence is 20 mJ/ ^mm² , the thickness of the passivation film in the laser-treated area of the stainless steel substrate separator obtained by the manufacturing method of the present invention is the same as before laser treatment, typically around 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., greater than 1 when the value before laser treatment is set to 1). In this embodiment, although the thickness of the passivation film in the laser-treated area 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 area is improved.

In another embodiment, when the laser fluence is 30 mJ/ ^mm² , in the fuel cell separator obtained by the manufacturing method of the present invention, the thickness of the passivation film in the laser-treated area on the surface of the stainless steel substrate increases compared to before laser treatment, and is typically 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 area 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.

The fuel cell separator of the present invention can have its corrosion resistance in corrosive environments improved by laser treatment. In this 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 with a potential of 0.8V to 1.0V for 24 hours (hr) or more.

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. Conversely, 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 entirely even in a corrosive environment.

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.

<Corrosion Test>
SUS304 stainless steel was used as the substrate. A portion of the surface of the SUS304 substrate was subjected to laser treatment with an energy density (fluence) of 20 mJ/ ^mm² or 30 mJ/ ^mm² and a processing speed of 3000 mm/s to obtain evaluation samples.

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. The presence or absence of corrosion (discoloration) after the corrosion test was visually confirmed.

In corrosion tests, the laser-treated samples showed no discoloration and no corrosion in either the 20 mJ/ ^mm² or 30 mJ/ ^mm² fluence evaluation samples. However, the untreated samples showed discoloration ranging from brown to purple, indicating corrosion. 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 samples is thought to be due to variations in the thickness and composition of the passive film on the surface of the SUS304 substrate.

<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; 187.85 eV, narrow; 46.95-117.40 eV), step size (wide; 0.4 eV, narrow; 0.1 eV), shift correction C1s; C-C, C-H, 284.8 eV. Depth analysis was performed while etching with Ar monomer at a voltage of 3kV, irradiation area of 2×2 mm, and etching speed (sputtering speed) of 6.5 nm/min.

1. Surface Analysis of Passivation Film Before Corrosion Testing 1-1. Elemental Analysis of the Outer Surface of Passivation Film Before Corrosion Testing Table 1 shows the results of the elemental analysis of the outer surface of the passivation film before corrosion testing. As shown in Table 1, in the low-fluence 20 mJ/ ^mm² laser treatment area, the amount of Cr and Mn increased and the amount of Fe decreased compared to the untreated area. On the other hand, in the high-fluence 30 mJ/ ^mm² laser treatment area, the amount of Cr decreased and the amount of Fe, Mn, and O increased compared to the untreated area. The Cr/Fe ratio of the laser-treated area was higher than that of the untreated area at the low-fluence 20 mJ/ ^mm² , but lower at the high-fluence 30 mJ/ ^mm² . On the other hand, the Mn/Fe ratio was higher in the laser-treated area than in the untreated area, regardless of fluence. An improvement in the Mn/Fe ratio indicates that the amount of Fe that binds with oxygen is relatively reduced in the passivated coating, and this is also thought to contribute to improved corrosion resistance.

1-2. Analysis of the passivation film in the depth direction before corrosion testing Figures 1A to C show the results of XPS analysis (sputtering rate: 6.5 nm/min) of the passivation film in the depth direction before corrosion testing (Figure 1A: 20 mJ/ ^mm² laser treatment, Figure 1B: 30 mJ/ ^mm² laser treatment, Figure 1C: untreated). The thickness of the passivation film (≒ oxide film) was determined from Figures 1A to C. Specifically, the thickness of the passivation film is generally defined as half of the maximum O amount (oxygen amount), 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 line arrows in Figures 1A to C). As a result, at a laser fluence of 20 mJ/ ^mm² , the thickness of the passivation film was approximately 2.3 nm (Figure 1A), and at a laser fluence of 30 mJ/ ^mm² , the thickness of the passivation film increased considerably to approximately 19.5 nm (Figure 1B). The thickness of the passivation film in the untreated area was approximately 2.0 nm (Figure 1C), which was consistent with the generally accepted thickness of the passivation film on stainless steel (1-2 nm).

From these findings, it was found that in the low-fluence 20 mJ/ ^mm² laser treatment area, corrosion resistance was improved despite the passivation film thickness being almost the same as in 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, in the 20 mJ/ ^mm² laser treatment area, the Cr and O spectra are sharper in the Cr₂O₃ range compared to the untreated area. This suggests that the passivation film is composed of a more Cr-rich oxide, which is likely the reason for the improved corrosion resistance. In the 30 mJ/ ^mm² laser treatment area, although the amount of Cr in the passivation film is small, it is presumed that the sharpness of the Cr and O spectra in the Cr₂O₃ range, similar to the 20 mJ/ ^mm² laser treatment area, and the thicker passivation film, contribute to the improved corrosion resistance. Thus, in the XPS spectrum representing the bonding state of Cr and O in the passivation film, it is considered important for the intensity of Cr and O to be sharp in the Cr2O3 region to improve corrosion resistance.

2. Surface Analysis of Passivated Coating After Corrosion Test 2-1. Elemental Analysis of the Outer Surface of the Passivated Coating After Corrosion Test Table 2 shows the results of the elemental analysis of the outer surface of the passivated 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 to 3C show the change in the Cr/Fe ratio for each corrosion test time in the laser-treated and untreated areas (Figure 3A: 20 mJ/ ^mm² laser-treated area, Figure 3B: 30 mJ/ ^mm² laser-treated area, Figure 3C: untreated area). From Figures 3A and 3B, the Cr/Fe ratio increased with corrosion test time in the laser-treated area. This means that in the laser-treated area, 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, as shown in Figure 3C, in the untreated SUS304 substrate, the Cr/Fe ratio on the passive coating surface decreased with the corrosion test time compared to the initial (before corrosion test, 0 hr) Cr/Fe ratio, eventually showing a tendency 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.

2-2. Depth-direction analysis of the passivation film after corrosion testing The results of 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 (sputtering rate: 6.5 nm/min) of the laser-treated area after corrosion testing (Figure 4A: 20 mJ/ ^mm² laser-treated area, Figure 4B: 30 mJ/ ^mm² laser-treated area). Figures 5A and 5B also show the results of XPS analysis of the passivation film in the depth direction (sputtering rate: 6.5 nm/min) of the untreated area (corroded area) after corrosion testing (Figure 5A: untreated area of the 20 mJ/ ^mm² laser-treated sample, Figure 5B: untreated area of the 30 mJ/ ^mm² 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 area (indicated by the broken line arrows (I) and (II) in Figures 4A and 4B), whereas from Figures 5A and 5B, the untreated area had changed into an oxide film with almost no Cr. As can be seen from the XPS spectra representing the Cr bonding state in the passive film after the corrosion test shown in Figure 6A, and the XPS spectra representing the O bonding state in the passive film after the corrosion test shown in Figure _6B , the laser-treated area had a _Cr₂O₃ passive film, while the untreated area had changed into an oxide film mainly _composed of _Fe₂O₃ .

Claims (3)

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).
A method for manufacturing a fuel cell separator, wherein the separator is modified into a passivation film in which the ratio of Cr content (atomic%) to Fe content (atomic%) (Cr/Fe ratio) 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. The method for manufacturing a fuel cell separator according to claim 1, wherein the energy density of the laser is 10 mJ/ ^mm² to 40 mJ/ ^mm² . The method for manufacturing a fuel cell separator according to claim 1 or 2, wherein the energy density of the laser is 10 mJ/ ^mm² to 20 mJ/ ^mm² .