Fuel cell bipolar plate coating material having high corrosion resistance and high conductivity characteristics, and coating method

Abstract

The present invention relates to a fuel cell bipolar plate having corrosion resistance and high conductivity, and a fuel cell bipolar plate coating method, and provides a fuel cell bipolar plate comprising: a metal substrate; and a coating layer which is formed on the metal substrate, and which is composed of a mixture of a binder resin and a filler comprising a flake-like carbon material and a granular carbon material, wherein the filler is encompassed by the binder resin so as to be dispersed inside the coating layer, and the filler is exposed to the outside on the surface of the coating layer.

Description

Coating material and coating method for fuel cell separator having high corrosion resistance and high conductivity properties

[0001] The present invention relates to a coating material and a coating method for a metal separator for a fuel cell that can secure conductivity while suppressing surface corrosion.

[0002] Recently, demand for electric vehicles (EVs) and hydrogen fuel cell vehicles (FCEVs) is increasing to address global warming, replacing internal combustion engines (ICEs). Among these, hydrogen fuel cell vehicles utilize a chemical reaction opposite to electrolysis. In other words, they are power generation systems that generate electricity (and heat) through the supply of hydrogen, which then drives a motor. While hydrogen fuel cells can theoretically generate 1.229 V, various limitations limit their operating voltage to 0.6–0.8 V. A hydrogen fuel cell stack consists of a bipolar plate, a gas diffusion layer (GDL), and a membrane electrode assembly (MEA) coated with catalyst powder.

[0003] The redox reaction equations of the fuel cell are as follows: (1) and (2).

[0004] (Hydrogen supply) H2→ 2H + + 2e - E 0 =0V vs SHE Electrochemical oxidation of hydrogen (Hydrogen Oxidative Reaction, HOR) … (1)

[0005] (Oxygen supply) 1 / 2O2+ 2H + + 2e - → H2O E 0 =1.229V vs SHE Electrochemical reduction of oxygen (Oxygen Reductive Reaction, ORR) … (2)

[0006] In addition to the high potential environment, the corrosive environment inside the fuel cell is a highly corrosive environment with low pH as shown in reactions (3) to (8). Radicals are generated by the reduction of metals (reactions 3 and 4), and SO4 is generated by the deterioration of the sulfonyl group and CF group of the resin. 2- , F - This is generated (Fenton Reaction) (Reaction Schemes 5 to 8).

[0007] Fe 2+ + H2O2→ Fe 3+ + ·OH + OH - … (3)

[0008] Fe 3+ + H2O2→ Fe 2+ + ·OOH + H + … (4)

[0009] R-SO4+ ·OH or ·OOH → ROH + SO4 2- … (5) → 2H2O + SO4 2- → 2H + + SO4 2- + 2OH - … (6)

[0010] RCF + ·OH or ·OOH → RC-OH + F - … (7) → H2O + F - → H + + F - + OH - … (8)

[0011] The separator is a key material that connects multiple unit cells and forms the framework of the stack, and a material that can withstand pH 4 or higher is required, and actual corrosion tests are being conducted in an environment of pH 1 to 3 and 0.6 to 0.0 V vs SCE. In such an environment, the material must exhibit a current density of 1 μA / cm2 or less at a constant potential of 0.6 V vs SCE, and an interfacial contact resistance of 10 mΩ㎠ or less (under 133 N / m pressure). In addition, the material conductivity is required to be 100 S / cm or higher, and although metallic materials exhibit a high initial conductivity of 104 or higher, there is a problem of the conductivity decreasing due to corrosion.

[0012] Titanium metal materials have excellent conductivity, but in a corrosive environment, the surface TiO2 is formed to be about 200 nm, which is much larger than the stainless passive film (5 nm) such as CrO3 or Cr3O7 of stainless steel, so its control is necessary (see Fig. 4). In other words, the partially passive growth type thin film of the stainless steel series cannot be applied to lightweight materials such as Ti, Al, and Mg. Existing thin film coatings such as physical vapor deposition (PVD) and chemical vapor deposition (CVD) have limitations in application due to coating time and economic issues. In addition, the thin film coating disclosed in Japanese Patent No. 06943781 is difficult to apply easily because it is applied through a vacuum-high temperature process.

[0013] The present invention aims to address these conventional problems by providing a fuel cell separator that has high external substance penetration resistance, thereby suppressing surface corrosion while ensuring conductivity. However, these tasks are exemplary and should not be construed as limiting the scope of the present invention.

[0014] According to one aspect of the present invention, a separator for a fuel cell is provided.

[0015] In one embodiment, a fuel cell separator comprises a metal substrate and a coating layer formed on the metal substrate and composed of a mixture of a binder resin and a filler including a flake-shaped carbon material and a granular carbon material, wherein the inside of the coating layer is such that the filler is dispersed and wrapped in the binder resin, and the surface of the coating layer is such that the filler is exposed to the outside.

[0016] In one embodiment, the filler may further comprise a metal powder.

[0017] In one embodiment, the filler may be at least one selected from the group consisting of graphene, carbon nanotubes, graphite, and carbon black.

[0018] In one embodiment, the thickness of the coating layer may be formed to be 0.01 to 10 μm.

[0019] In one embodiment, the density of the filler in the coating layer is 10 to 10 4 It can be EA / ㎠.

[0020] In one embodiment, the filler may be formed at an angle of 30 to 90 degrees with respect to the substrate surface.

[0021] In one embodiment, the metal substrate may be formed of titanium or a titanium alloy.

[0022] In one embodiment, the binder resin may be a linear or branched polymer.

[0023] In one embodiment, the metal powder may be stainless steel powder.

[0024] According to another aspect of the present invention, a method for coating a separator for a fuel cell is provided.

[0025] In one embodiment, the method for coating a separator for a fuel cell includes the steps of mixing 10 to 70 wt% of a binder resin and 30 to 90 wt% of a filler composed of a flake-shaped carbon material and a granular carbon material, applying the mixture on a substrate and thermally curing it, and brushing the cured substrate surface; wherein the flake-shaped carbon material and the granular carbon material may be mixed in a weight ratio of 7:3 to 8:2.

[0026] In one embodiment, the filler further comprises a metal powder, and may be a mixture of 60 to 80 wt% of flake carbon material, 20 to 30 wt% of granular carbon material, and 0.01 to 10 wt% of metal powder.

[0027] In one embodiment, the filler may be at least one selected from the group consisting of graphene, carbon nanotubes, graphite, and carbon black.

[0028] In one embodiment, the substrate may be formed of titanium or a titanium alloy.

[0029] In one embodiment, the binder resin may be a linear or branched polymer.

[0030] In one embodiment, the metal powder may be stainless steel powder.

[0031] According to the embodiments of the present invention, which are achieved as described above, the durability of the stack is improved by providing a highly corrosion-resistant and highly conductive separator. Furthermore, by providing a paint-type coating method, it can replace existing deposition-type and electroplating-type coatings and secure cost-effectiveness compared to existing methods. Furthermore, it can be applied to coating automotive, aerospace, and stationary PEMFC separator plates, thereby broadening the range of separator material choices.

[0032] Of course, the scope of the present invention is not limited by these effects.

[0033] Figure 1 is a schematic diagram showing the structure of a coating layer according to an embodiment of the present invention.

[0034] Figure 2 is a schematic diagram showing the state of filler in a coating layer according to an embodiment of the present invention.

[0035] Figure 3 is a graph comparing stack performance changes according to the type of metal substrate in an embodiment of the present invention.

[0036] Figure 4 is a photograph comparing the corrosion environment after heat treatment according to the type of metal substrate in an embodiment of the present invention.

[0037] Hereinafter, various embodiments of the present invention will be described in detail with reference to the attached drawings. These embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art. The following embodiments may be modified in various different forms, and the scope of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to more faithfully and completely explain the present disclosure and to fully convey the spirit of the present invention to those skilled in the art. In addition, the thickness and size of each layer in the drawings are exaggerated for convenience and clarity of explanation.

[0038] Figure 1 shows a separator for a fuel cell according to an embodiment of the present invention.

[0039] In Fig. 1, a coating layer is formed on a metal substrate (10), and the coating layer is composed of a mixture of a binder resin (20) and a filler (30) including a flake-shaped carbon material (32) and a granular carbon material (31). The coating layer can be formed by applying a composition containing 10 to 70 wt% of the binder resin (20) and 30 to 90 wt% of the filler (30) composed of the flake-shaped carbon material and the granular carbon material onto the substrate. The method of applying the composition may use a pre-coating method, a nano-spraying method, etc., but is not limited thereto.

[0040] The binder resin (20) binds the filler (30) composed of carbon material and is an important factor in determining the coating properties. If the binder resin content exceeds 70 wt%, the surface resistance becomes excessively high, and if it is lower than 10 wt%, the mechanical properties of the coating cannot be secured. Therefore, it is preferable to mix it in a ratio of 10 to 70 wt%.

[0041] The type of binder resin (20) may include, but is not limited to, conventional resins such as polyacylic, polyphenolic, and polyester, as well as conductive resins such as polyaniline, polyphenylene surlfide (PPS), and polyacetylene. The binder resin composition may be blended with various components, such as solvents, curing agents, pigments, pigment dispersants, and additives. A polymerization initiator such as a thermal initiator or a photoinitiator may be further included to facilitate polymerization.

[0042] The binder resin composition as described above can be subjected to a heat curing or photo-curing process to produce a binder resin (20). For example, the curing reaction may be a solid polymerization using a blocked isocyanate. The curing reaction may occur as a blocking agent such as a ketone or alcohol is dissociated. In the case of a heat curing reaction, the reaction may proceed at 20 to 250 degrees. The conductivity of the binder resin (20) after curing is 10 -4 It can have a range of 10 S / cm.

[0043] The binder resin can be a linear or branched polymer. The more branched the polymer, the higher its corrosion resistance, but the lower its conductivity. Therefore, in embodiments of the present invention, linear polymers can be used, but branched polymers can also be appropriately mixed. For example, branching can be introduced by grafting monomers onto the linear polymer backbone.

[0044] The binder resin (20) can be adjusted in molecular weight and viscosity to uniformly encapsulate the fillers and prevent them from settling and remain in a uniformly dispersed state. Since the dispersibility of the filler is important in determining the conductivity of the separator, it is necessary to uniformly disperse the filler in the binder resin. Since the viscosity of the mixture increases as the molecular weight of the binder resin increases, it is recommended that the molecular weight of the binder resin be adjusted within the range of 100 to 5000 MW.

[0045] Organic coatings, while offering excellent corrosion resistance, suffer from high resistance due to poor resin resistance and poor interconnectivity between conductive fillers. Therefore, in an embodiment of the present invention, a mixture of binder resin and filler is applied to a substrate and then heat-cured, thereby reducing the coating thickness through heat shrinkage and allowing the fillers to adhere tightly. The coating layer thickness is set to 10 μm or less, thereby suppressing the increase in resistance associated with increasing coating thickness.

[0046] It is important to control the type and orientation of the filler (30) to lower the surface resistance of the separator. The filler (30) is composed of carbon materials such as graphene, carbon nanotubes (CNTs), graphite, and carbon black, and may further include metal powder as needed. The shape of the filler can affect the formation of a network for electron movement. In one embodiment, highly conductive flake-shaped graphene or carbon nanotubes can be used to form a needle-like or flaky structure for contact between numerous points and surfaces.

[0047] To prevent flake-shaped carbon materials from separating from each other due to the binder resin, thereby reducing conductivity, granular graphite or carbon black may be mixed. The granular carbon material is partially embedded into the surface of the flake-shaped carbon material, thereby reducing the distance between the flake-shaped carbon materials, thereby forming a network path for electron transport. The flake-shaped carbon material and the granular carbon material may be mixed in a weight ratio of 7:3 to 8:2.

[0048] In another embodiment, metal powder may be added to enhance the conductivity of the filler. The metal powder may be, but is not limited to, stainless steel or a precious metal. For example, the conductivity of the filler may be enhanced by mixing 60 to 80 wt% of flake carbon material, 20 to 30 wt% of granular carbon material, and 0.01 to 10 wt% of metal powder.

[0049] In order to lower the surface resistance of the separator, the flake-shaped carbon material constituting the filler (30) may be oriented at an angle of 30 to 90 degrees, more preferably 40 to 90 degrees, with respect to the surface of the metal substrate (10). If the filler is arranged randomly or oriented at a low angle, the flow of current may become uneven and the flow of current may be partially cut off by the polymer resin having insulating performance, which may increase the contact resistance and reduce the electrical conductivity. In one embodiment, a step of coating a composition including the filler on a substrate and then aligning the filler at an angle of 30 degrees or more may be further included. For example, during the coating process, a magnetic field or electric field generator may be connected to a coater to generate a magnetic field or electric field around the filler to align the direction of the filler, or a physical force may be applied to the coating layer to align the filler at an angle of 30 degrees or more. Through this, the electric flow in the coating layer can be smoothly controlled.

[0050] In one embodiment, the density of the filler is 10 to 10 4 EA / ㎠, more preferably 100 to 10 4 High conductivity and low contact resistance can be achieved by making it EA / ㎠. The viscosity of the paste containing carbon material or carbon material and metal mixture is 10 3 10 inland 4 It could be S / cm.

[0051] A fuel cell separator manufactured according to an embodiment of the present invention may exhibit a conductivity of 500 S / cm or more, and have a contact resistance value of 35 mΩ / cm2 or less in a state where no pressure is applied, and 5 mΩ / cm2 or less in a state where a pressure of 133 N / cm2 is applied.

[0052] The amount of filler mixed with the binder resin is preferably 30 to 90 wt%. If the filler content is less than 30 wt%, it is difficult to expect an improvement in conductivity because a network path for electron movement cannot be formed. If it exceeds 90 wt%, the filler dispersibility deteriorates and there is a problem of reduced molding processability.

[0053] Fig. 2 is a schematic diagram illustrating a step of exposing a filler by brushing the surface of a coating layer. The filler can be exposed to the outside by removing the binder resin that surrounds the filler on the surface of the coating layer. For example, the number of brushing cycles can be 300 to 2000. Fig. 2(a) shows that the filler is exposed to the surface of the coating layer through brushing, thereby improving conductivity. Fig. 2(b) shows that by removing a portion of the coating layer, some of the binder resin located in the outermost layer is removed, thereby increasing the filler area. As a result, the filler exists inside the coating layer in a state where it is surrounded and dispersed by the binder resin, and the filler exists on the surface of the coating layer in a state where it is exposed to the outside.

[0054] The metal substrate (10) can be formed of titanium or a titanium alloy. Titanium (Ti) material is more expensive than stainless steel (Stainless) material, but can exhibit high stack performance. When using the same membrane electrode assembly (MEA) and at a current density of 1 A / ㎠, the absolute ratio of the performance (Power) of the separator using stainless steel and Ti materials is as shown in Fig. 3. After 100 cycles, stainless steel shows an output of 0.6897, and Ti shows an output of 0.8379, so the durability performance of the titanium material is superior. In addition, since the specific gravity of stainless steel is 7.9, while that of Ti is low at about 4.6, a weight reduction of more than 40% is possible when applied with the same thickness.

[0055] As described above, the fuel cell separator according to the embodiment of the present invention can have excellent corrosion resistance, low contact resistance, and excellent electrical conductivity.

[0056] While the present invention has been described with reference to the embodiments illustrated in the drawings, these are merely exemplary, and those skilled in the art will appreciate that various modifications and equivalent alternative embodiments are possible. Therefore, the true scope of technical protection of the present invention should be determined by the technical spirit of the appended claims.

[0057] (Explanation of symbols)

[0058] 10: Metal substrate

[0059] 20: Binder resin

[0060] 30: Filler

[0061] 31: Granular carbon material

[0062] 32: Flake-shaped carbon material

[0063] The present invention can be utilized in the field of fuel cells, the field of metal separators for fuel cells, and the field of metal separator coating and coating materials, and can improve the reliability and competitiveness of products.

Claims

1. A fuel cell separator coating method, comprising:mixing 10 to 70 wt% of a binder resin and 30 to 90 wt% of a filler consisting of a flake-like carbon material and a granular carbon material;a coating step of applying a mixture on a substrate and performing heat curing; andbrushing a surface of a cured coating layer on the substrate to expose the filler existing on the surface of the coating layer to an outside;wherein the flake-like carbon material and the granular carbon material are mixed with each other in a weight ratio of 7:3 to 8:2, andwherein the coating step comprises generating a magnetic or electric field around the filler, or applying a physical force, such that the filler is orientated at an angle of 30 to 90 degrees with respect to a surface of the substrate.

2. The fuel cell separator coating method of claim 1, wherein the filler is at least one selected from the group consisting of graphene, carbon nanotubes, graphite, and carbon black.2022462805   24 Aug 2026

3. The fuel cell separator coating method of claim 1 or claim 2, wherein the substrate is formed of titanium or a titanium alloy.

4. The fuel cell separator coating method of any one of claims 1 to 3, wherein the binder resin is a linear or branched polymer.

5. The fuel cell separator coating method of any one of claims 1 to 4, wherein the filler further comprises a metal powder, the metal powder is a stainless steel powder.

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