Separator for fuel cell

Abstract

A separator (20) for a fuel cell comprises: a metal substrate (30); an intermediate layer (41); a metal carbide layer (42); and a conductive coating (43). The conductive coating (43) is made of conductive carbon. The metal carbide layer (42) is interposed between the intermediate layer (41) and the conductive coating (43). The metal carbide layer (42) is made of a carbide of a metal element contained in the intermediate layer (41). The thickness (D1) of the metal carbide layer (42) is 10 nm or less. Alternatively, when a scratch test is performed on the conductive coating (43), the value obtained by dividing a horizontal load (RH) at which peeling occurs in the conductive coating (43) by a vertical load (RV) is less than 0.35.

Description

Separator for fuel cell

[0001] The present disclosure relates to a separator for a fuel cell.

[0002] A unit cell of a fuel cell stack is configured by sandwiching a membrane electrode assembly between a pair of separators for fuel cells. When operating the fuel cell stack, a reaction gas containing moisture and a cooling medium such as cooling water come into contact with the separator for fuel cell. For this reason, corrosion resistance is required for separators for fuel cells.

[0003] Accordingly, it has conventionally been proposed to form a fuel cell separator by forming a conductive coating exhibiting corrosion resistance on a metal base material. For example, Japanese Patent No. 5476328 discloses a technique of forming a coating layer containing metal and carbon as a conductive coating. In the coating layer, the metal and carbon have a concentration gradient. Specifically, in the coating layer, the concentration of the metal increases as approaching the metal base material, while the concentration of carbon increases as moving away from the metal base material.

[0004] When a fuel cell stack is operated over a long period of time, pitting corrosion may occur particularly in the metal base material of the separator.

[0005] An object of the present disclosure is to solve the above-described problem.

[0006] A first aspect of the present disclosure is a separator for a fuel cell comprising a metal base material, an intermediate layer formed on the metal base material, and a conductive coating formed on the intermediate layer, wherein a material of the intermediate layer is a metal or a metal compound, a material of the conductive coating is conductive carbon, and between the intermediate layer and the conductive coating, there is provided a metal carbide layer made of a carbide of a metal element contained in the intermediate layer and having a film thickness of 10 nm or less.

[0007] A second aspect of the present disclosure is a fuel cell separator comprising a metal substrate, an intermediate layer formed on the metal substrate, and a conductive coating formed on the intermediate layer, wherein the material of the intermediate layer is a metal or a metal compound, and the material of the conductive coating is conductive carbon, and the value obtained by dividing the horizontal load by the vertical load when peeling occurs in the conductive coating during a scratch test on the conductive coating is less than 0.35.

[0008] According to this disclosure, pitting corrosion in fuel cell separators can be avoided.

[0009] Figure 1 is a cross-sectional view of the main part of a unit cell constituting a fuel cell stack. Figure 2 is an enlarged cross-sectional view of the main part of a fuel cell separator. Figure 3 is a chart showing the relationship between the main physical properties of the coating layer and the presence or absence of pitting corrosion or delamination. Figure 4 is an electron microscope image of a separator in which pitting corrosion has occurred. Figure 5 is a graph showing the relationship between the film thickness of the metal carbide layer and the horizontal load in the scratch test. Figure 6 is an electron microscope image of a separator in which no pitting corrosion was observed.

[0010] Hereafter, fuel cell separators may be simply referred to as "separators." In the scratch test described below, an MTS Systems Nanoindenter G200 is used, and a diamond cone-shaped indenter is pressed against the sample at 23°C. Furthermore, the indenter is scanned horizontally for 280 μm in this state, and the vertical load (indentation load) at which delamination occurs is evaluated as the delamination strength.

[0011] Figure 1 is a cross-sectional view of the main part of a unit cell 10 that constitutes a fuel cell stack (not shown). Note that the dimensions of each component shown in Figure 1 do not necessarily reflect the actual dimensions.

[0012] The unit cell 10 comprises a membrane electrode structure 12 and a pair of separators 20. The membrane electrode structure 12 has an electrolyte membrane 14, an anode electrode 16 provided on one end face of the electrolyte membrane 14, and a cathode electrode 18 provided on the other end face of the electrolyte membrane 14. The anode electrode 16 has a first gas diffusion layer 161 and a first electrode catalyst layer 162, and the cathode electrode 18 has a second gas diffusion layer 181 and a second electrode catalyst layer 182. The electrolyte membrane 14 is interposed between the first electrode catalyst layer 162 and the second electrode catalyst layer 182.

[0013] A pair of separators 20 includes an anode-side separator 22 and a cathode-side separator 24. The anode-side separator 22 has a fuel gas flow path 221 and a first cooling medium flow path 222. The cathode-side separator 24 has an oxidizer gas flow path 241 and a second cooling medium flow path 242. In a fuel cell stack formed by stacking unit cells 10, the first cooling medium flow path 222 of the anode-side separator 22 and the second cooling medium flow path 242 of the cathode-side separator 24 are in communication, forming a single cooling medium flow path.

[0014] As shown in Figure 2, the separator 20 (anode-side separator 22 or cathode-side separator 24) has a metal substrate 30 and a coating layer 40. The coating layer 40 is composed of an intermediate layer 41, a metal carbide layer 42, and a conductive film 43, which are laminated together. The intermediate layer 41 is the innermost layer of the coating layer 40 and is formed on the surface of the metal substrate 30. The conductive film 43 is the outermost layer of the coating layer 40. The metal carbide layer 42 is interposed between the intermediate layer 41 and the conductive film 43. As can be understood from this, the conductive film 43 is formed on the metal substrate 30 via the intermediate layer 41 and the metal carbide layer 42.

[0015] The material of the metal substrate 30 is preferably a metal that exhibits sufficient corrosion resistance and has low electrical resistance. A specific example of such a metal is stainless steel. Since stainless steel is inexpensive, it can also help reduce material costs. However, the material of the metal substrate 30 is not limited to stainless steel.

[0016] The intermediate layer 41 is preferably formed from a material that exhibits excellent bonding strength to the metal substrate 30 and also excellent bonding strength to the conductive coating 43. Furthermore, the thermal expansion coefficient of this material is preferably between the thermal expansion coefficient of the metal substrate 30 and the thermal expansion coefficient of the conductive coating 43. As will be described later, the material of the conductive coating 43 is conductive carbon. As mentioned above, a suitable example of the metal substrate 30 is stainless steel. Specific examples of materials that exhibit sufficient bonding strength to the conductive coating 43 and the metal substrate 30 and satisfy the above-mentioned thermal expansion coefficient requirements include metals such as Ti (titanium), Cr (chromium), Nb (niobium), or Sn (tin). The material of the intermediate layer 41 may also be a metal compound such as CrN (chromium nitride). However, the material of the intermediate layer 41 is not limited to these metals or metal compounds (metal nitrides).

[0017] The material of the conductive coating 43 is conductive carbon. Specific examples of conductive carbon include carbon containing multiple graphite structures. The material of the conductive coating 43 may also be polycrystalline graphite, glassy carbon, or amorphous carbon, etc.

[0018] The thickness D2 of the conductive coating 43 configured as described above is, for example, 30 nm to 200 nm, preferably 50 nm to 80 nm. In this case, the conductive coating 43 adheres well to the intermediate layer 41 via the metal carbide layer 42. Furthermore, since the thickness of the coating layer 40 is avoided to an excessive degree, it is possible to avoid long film formation times and soaring manufacturing costs. However, the thickness D2 of the conductive coating 43 is not limited to the range of 30 nm to 200 nm.

[0019] The metal carbide layer 42 contains a metal element of the metal or metal compound that is the material of the intermediate layer 41, and carbon. The metal and carbon chemically bond to form a metal carbide. In other words, the material of the metal carbide layer 42 is a metal carbide. If the material of the intermediate layer 41 is Ti and Ti combines with carbon, then the metal carbide layer 42 is a TiC film.

[0020] The coating layer 40 can be obtained by various known film formation methods. Specific examples of film formation methods include vapor deposition, sputtering, ion plating, and chemical vapor deposition.

[0021] Here, we will describe the results of scratch tests performed on samples having a coating layer 40 formed on the surface of a metal substrate 30 by changing various film formation conditions. In all samples, the material of the intermediate layer 41 is Ti, and the material of the conductive film 43 is polycrystalline graphite containing multiple graphite structures.

[0022] In the scratch test, the horizontal load at which delamination occurred in the conductive coating 43 was defined as RH, and the vertical load at which delamination occurred in the conductive coating 43 was defined as RV. The value obtained by dividing the horizontal load RH by the vertical load RV (RH / RV) was then calculated. On the other hand, the same sample used in the scratch test was subjected to an acid resistance test in which it was left in a predetermined acidic environment to check whether pitting corrosion occurred. As a result, as shown in Figure 3, when RH / RV was 0.35 or higher, pitting corrosion was observed in the metal substrate 30. Figure 4 shows an electron microscope image of the separator 20 in which pitting corrosion occurred.

[0023] Figure 5 is a graph showing the relationship between the film thickness D1 of the metal carbide layer 42 (see Figure 2) and the horizontal load RH in the scratch test. As can be seen from Figure 5, when the film thickness D1 of the metal carbide layer 42 was 10 nm or less, no pitting corrosion was observed in the coating layer 40 and the metal substrate 30. Figure 6 shows an electron microscope image of the separator 20 in which no pitting corrosion was observed.

[0024] The reason why pitting corrosion is not observed when the RH / RV is less than 0.35, or when the film thickness D1 of the metal carbide layer 42 is 10 nm or less, is presumed to be because the conductive coating 43 becomes more easily peeled off from the intermediate layer 41 or the metal carbide layer 42 under certain conditions. In other words, microscopic defects such as microcracks occur in the conductive coating 43. If acid seeps out from the electrolyte membrane 14 while such a conductive coating 43 remains without peeling off from the intermediate layer 41 or the metal carbide layer 42, it is thought that the acid reaches the metal substrate 30 through the microcracks. In this case, the acid seeping out from the electrolyte membrane 14 has difficulty passing through the metal carbide layer 42.

[0025] For the reasons stated above, in this embodiment, the coating layer 40 is formed such that the RH / RV is less than 0.35 and the film thickness D1 of the metal carbide layer 42 (see Figure 2) is 10 nm or less. That is, the film formation conditions for the intermediate layer 41, the metal carbide layer 42, and the conductive film 43 are set so that the above-mentioned coating layer 40 is obtained. As a result, a separator 20 in which pitting corrosion is not observed is obtained, as shown in Figure 6. It is preferable that the film thickness D1 is 1 nm or more.

[0026] In the illustrated example, the separator 20 is shown to have a metal carbide layer 42 with a film thickness D1 of 10 nm or less and an RH / RV of less than 0.35. However, the separator 20 according to this embodiment includes an embodiment in which the separator 20 has a metal carbide layer 42 with a film thickness D1 of 10 nm or less and an RH / RV of 0.35 or more, and an embodiment in which the separator 20 has a metal carbide layer 42 with a film thickness D1 exceeding 10 nm and an RH / RV of less than 0.35. In other words, the separator 20 only needs to satisfy either having a metal carbide layer 42 with a film thickness D1 of 10 nm or less, or having an RH / RV of less than 0.35.

[0027] If the RH / RV ratio is less than 0.35, the coating layer 40 does not need to include the metal carbide layer 42. In other words, in this case, the coating layer 40 may be composed of the intermediate layer 41 and the conductive film 43.

[0028] In this configuration, the electrolyte membrane 14 in the unit cell 10 of the fuel cell stack (see Figure 1) is typically made of perfluorocarbon sulfonic acid. When the electrolyte membrane 14 deteriorates over time due to the operation of the fuel cell stack, it is expected that a small amount of acid will seep out from the electrolyte membrane 14. When this acid comes into contact with the separator 20, the conductive coating 43 (see Figure 2), which has low peel strength, peels off relatively easily from the intermediate layer 41 or the metal carbide layer 42. Figure 3 also shows whether or not peeling occurred in an acid test in which the separator 20 was left in a predetermined acidic environment.

[0029] As shown in Figure 3, in separators 20 where the peel strength of the conductive coating 43 is less than 8 mN, peeling is observed in the acid resistance test. Therefore, it is preferable that the peel strength of the conductive coating 43 in the separator 20 be 8 mN or more. This suppresses the peeling of the conductive coating 43 (see Figure 2) from the intermediate layer 41 or the metal carbide layer 42, even when the fuel cell stack is operated for a long period of time. As a result, the metal substrate 30 is protected from acid for a long period of time, and corrosion of the metal substrate 30 by acid is avoided.

[0030] The effects of this embodiment can be summarized as follows:

[0031] As shown in Figures 1 and 2, the separator 20 comprises a metal substrate 30 and a conductive coating 43 formed on the metal substrate 30 via an intermediate layer 41. A metal carbide layer 42 is interposed between the intermediate layer 41 and the conductive coating 43. The metal carbide layer 42 contains the metal elements contained in the intermediate layer 41 and carbon. The film thickness D1 of the metal carbide layer 42 is 10 nm or less.

[0032] Alternatively, when the conductive film 43 peels off during a scratch test on the coating layer 40, the RH / RV ratio is less than 0.35.

[0033] If at least one of the conditions is met, pitting corrosion on the separator 20 (especially the metal substrate 30) can be avoided.

[0034] The peeling load of the conductive coating 43 in the scratch test is preferably 8 mN or more. This prevents the conductive coating 43 from peeling off even when the fuel cell stack is operated for a long period of time. Consequently, corrosion of the metal substrate 30 by acid is suppressed.

[0035] The material of the intermediate layer 41 is preferably Ti. In this case, the metal carbide layer 42 contains Ti. This configuration makes it possible to reduce the manufacturing cost of the separator 20.

[0036] The material of the metal substrate 30 is preferably stainless steel. Since stainless steel is inexpensive, it can help reduce the manufacturing cost of the separator 20.

[0037] The following additional information is disclosed regarding the above embodiments.

[0038] (Note 1) The fuel cell separator (20) of the present disclosure comprises a metal substrate (30), an intermediate layer (41) formed on the metal substrate, and a conductive coating (43) formed on the intermediate layer, wherein the material of the intermediate layer is a metal or a metal compound, and the material of the conductive coating is conductive carbon, and a metal carbide layer (42) is provided between the intermediate layer and the conductive coating, the material of which is a carbide of a metal element contained in the intermediate layer and the thickness (D1) is 10 nm or less.

[0039] This configuration prevents pitting corrosion from occurring in the fuel cell separator.

[0040] (Note 2) In the fuel cell separator described in Note 1, the vertical load on the conductive coating in the scratch test may be 8 mN or more.

[0041] This configuration prevents the conductive coating from peeling off.

[0042] (Note 3) In the fuel cell separator described in Note 1 or 2, the material of the intermediate layer is Ti, and the metal carbide layer may also contain Ti.

[0043] In this case, the manufacturing cost of fuel cell separators can be reduced.

[0044] (Supplementary Note 4) In the fuel cell separator according to any one of Supplementary Notes 1 to 3, the material of the metal base material may be stainless steel.

[0045] In this case, the manufacturing cost of the fuel cell separator can be reduced.

[0046] (Supplementary Note 5) A fuel cell separator (20) of the present disclosure includes a metal base material (30), an intermediate layer (41) formed on the metal base material, and a conductive coating (43) formed on the intermediate layer, wherein a material of the intermediate layer is a metal or a metal compound, a material of the conductive coating is conductive carbon, and a value obtained by dividing a horizontal load (RH) by a vertical load (RV) when peeling of the conductive coating occurs in a scratch test on the conductive coating is less than 0.35.

[0047] According to this configuration, occurrence of pitting corrosion in the fuel cell separator can be avoided.

[0048] (Supplementary Note 6) In the fuel cell separator according to Supplementary Note 5, a peeling load of the conductive coating in the scratch test may be 8 mN or more.

[0049] According to this configuration, peeling of the conductive coating can be avoided.

[0050] (Supplementary Note 7) In the fuel cell separator according to Supplementary Note 5 or 6, a metal carbide layer (42) made of a carbide of a metal element contained in the intermediate layer and having a film thickness (D1) of 10 nm or less may be provided between the intermediate layer and the conductive coating.

[0051] According to this configuration, occurrence of pitting corrosion in the fuel cell separator can be further avoided.

[0052] (Supplementary Note 8) In the fuel cell separator according to any one of Supplementary Notes 5 to 7, the material of the intermediate layer may be Ti.

[0053] In this case, the manufacturing cost of the fuel cell separator can be reduced.

[0054] (Note 9) In the fuel cell separator described in any one of Notes 5 to 8, the material of the metal substrate may be stainless steel.

[0055] In this case, the manufacturing cost of fuel cell separators can be reduced.

[0056] While this disclosure has been described in detail, it is not limited to the individual embodiments described above. These embodiments can be added, replaced, modified, partially deleted, etc., in any way that does not depart from the gist of this disclosure or from the intent of this disclosure derived from the claims and their equivalents. These embodiments can also be implemented in combination. For example, the order of operations and processes in the embodiments described above are given as examples only and are not limited thereto. The same applies when numerical values ​​or mathematical formulas are used in the description of the embodiments described above.

[0057] 10... Unit cell 12... Membrane electrode structure 14... Electrolyte membrane 20... Separator 30... Metal substrate 40... Coating layer 41... Intermediate layer 42... Metal carbide layer 43... Conductive coating

Claims

1. A fuel cell separator (20) comprising a metal substrate (30), an intermediate layer (41) formed on the metal substrate, and a conductive coating (43) formed on the intermediate layer, wherein the intermediate layer is made of a metal or a metal compound, and the conductive coating is made of conductive carbon, and a metal carbide layer (42) is provided between the intermediate layer and the conductive coating, the metal carbide layer (42) is made of a carbide of a metal element contained in the intermediate layer and has a film thickness (D1) of 10 nm or less.

2. A fuel cell separator according to claim 1, wherein the vertical load of the conductive coating in a scratch test is 8 mN or more.

3. A fuel cell separator according to claim 1 or 2, wherein the material of the intermediate layer is Ti, and the metal carbide layer contains Ti.

4. A fuel cell separator according to claim 1 or 2, wherein the material of the metal substrate is stainless steel.

5. A fuel cell separator (20) comprising a metal substrate (30), an intermediate layer (41) formed on the metal substrate, and a conductive coating (43) formed on the intermediate layer, wherein the material of the intermediate layer is a metal or a metal compound, and the material of the conductive coating is conductive carbon, and the value obtained by dividing the horizontal load (RH) by the vertical load (RV) when peeling occurs in the conductive coating during a scratch test on the conductive coating is less than 0.

35.

6. A fuel cell separator according to claim 5, wherein the peeling load of the conductive coating in the scratch test is 8 mN or more.

7. A fuel cell separator according to claim 5, comprising a metal carbide layer (42) between the intermediate layer and the conductive coating, the metal carbide layer (42) being made of a carbide of a metal element contained in the intermediate layer and having a thickness of 10 nm or less.

8. A fuel cell separator according to any one of claims 5 to 7, wherein the material of the intermediate layer is Ti.

9. A fuel cell separator according to any one of claims 5 to 7, wherein the material of the metal substrate is stainless steel.

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