Coating material for fuel cell separator plates having high corrosion resistance and high conductivity characteristics, and coating method

Abstract

The present invention relates to a separator for a fuel cell having corrosion resistance and high conductivity, and a method for coating the separator for a fuel cell, which includes a metal substrate and a coating layer formed on the metal substrate and composed of a mixture of a binder resin and a filler containing a flake-shaped carbon material and a granular carbon material. Inside the coating layer, the filler exists in a state of being dispersed while being wrapped by the binder resin, and on the surface of the coating layer, a separator for a fuel cell is provided in a state where the filler is exposed to the outside.

Description

Technical Field

[0001] The present invention relates to a coating material for a metal separator plate for a fuel cell and a coating method thereof, which can suppress surface corrosion while ensuring conductivity.

Background Art

[0002] In recent years, in order to address global warming, the demand for electric vehicles (EVs) and hydrogen fuel cell vehicles (FCEVs) that replace internal combustion engines (ICEs) has been increasing. Among them, hydrogen fuel cell vehicles have a chemical reaction opposite to electrolysis. That is, it is a system that generates electricity (and heat) through a power generation system using hydrogen supply and drives a motor. In theory, a hydrogen fuel cell can generate a voltage of 1.229 V, but due to various limiting conditions, it has an operating voltage of 0.6 to 0.8 V. A hydrogen fuel cell stack is composed of a bipolar plate, a gas diffusion layer (GDL), and a membrane electrode assembly (MEA) coated with a catalyst powder.

[0003] The redox reaction formula of the fuel cell is as shown in (1) and (2).

[0004] (Hydrogen supply section) H2 → 2H + + 2e - E 0 = 0V vs SHE Hydrogen electrochemically oxidative reaction (Hydrogen Oxidative Reaction, HOR) … (1)

[0005] (Oxygen supply section) 1 / 2O2 + 2H + + 2e - → H2O E 0 = 1.229V vs SHE Oxygen electrochemically reductive reaction (Oxygen Reductive Reaction, ORR) … (2)

[0006] In addition to the high-potential environment, the corrosion environment inside the fuel cell has a low pH and a high corrosion environment, as shown in Reaction Formulas (3) to (8). Due to the reduction of metal, radicals are generated (Reaction Formulas 3 and 4), and due to the deterioration of the sulfonyl group and C-F group of the resin, SO4 2- , F - is generated (Fenton Reaction) (Reaction Formulas 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] R-C-F+·OH or ·OOH→R-C-OH+F - … (7)→H2O+F - →H + +F - +OH - … (8)

[0011] The separator plate connects multiple unit cells and is the core material that constitutes the stack skeleton. A material that can withstand a pH of 4 or higher is required. In actual corrosion tests, experiments are also being carried out in environments with a pH of 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 / cm 2 or less at the same potential of 0.6 V vs SCE, and the interfacial contact resistance is 10 mΩcm 2It must represent the following (under a pressure of 133 N / m). In addition, the conductivity of the material is required to be 100 S / cm or more. Although metal materials exhibit a high conductivity of 104 or more initially, there is a disadvantage that the conductivity decreases due to corrosion.

[0012] In the case of titanium metal materials, although they have excellent conductivity, in a corrosive environment, a TiO2 layer of about 200 nm is formed on the surface layer. This is much larger than the passive films (5 nm) of stainless steel such as CrO3 and Cr3O7 of stainless steel, so its control is required (see Figure 4). That is, the partial passive growth type thin films 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 their application due to coating time and economic problems. In addition, the thin film coating disclosed in Japanese Patent No. 06943781 is applied to a vacuum-high temperature process and is difficult to apply easily.

Summary of the Invention

Problems to be Solved by the Invention

[0013] The present invention is for solving such conventional problems, and an object thereof is to provide a separator for a fuel cell that has a high external substance permeation resistance and can ensure conductivity while suppressing surface corrosion. However, such problems are exemplary and do not limit the scope of the present invention.

Means for Solving the Problems

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

[0015] In one embodiment, the separator for a fuel cell includes a metal substrate and a coating layer formed on the metal substrate and composed of a mixture of a binder resin and a filler containing flake-shaped carbon material and granular carbon material. Inside the coating layer, the filler exists in a state of being dispersed while wrapped by the binder resin, and on the surface of the coating layer, the filler can exist in a state of being exposed to the outside.

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

[0017] In one embodiment, the filler may be at least one or more 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 may be 10 to 10 4 EA / cm 2 and may be.

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

[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 coating method for the separator plate for the fuel cell includes a step of mixing 10 to 70 wt% of a binder resin and 30 to 90 wt% of a filler composed of a flake-like carbon material and a granular carbon material, a step of applying the mixture onto a substrate and thermally curing it, and a step of brushing the surface of the cured substrate; and the flake-like 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 may further include a metal powder and may be mixed with 60 to 80 wt% of a flake-like carbon material, 20 to 30 wt% of a granular carbon material, and 0.01 to 10 wt% of a metal powder.

[0027] In one embodiment, the filler may be at least one or more 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.

Advantages of the Invention

[0031] According to the embodiments of the present invention made as described above, by providing a separator plate with high corrosion resistance and high conductivity, the durability of the stack is improved. In addition, by providing a paint-type coating method, the existing vapor deposition type and electroplating type coatings are replaced, and economic efficiency can be ensured as compared with the existing ones. Further, it can be applied to the coating of separator plates for automotive, aviation, and stationary PEMFCs, and has the effect of widening the range of material selection for separator plates.

[0032] Of course, such effects do not limit the scope of the present invention.

Brief Description of the Drawings

[0033]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0034] Hereinafter, various embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more fully explain the present invention to those having ordinary knowledge in the technical field. The following embodiments can be modified into various different forms, and the scope of the present invention is not limited to the following embodiments. Rather, those embodiments are provided to make the present disclosure more substantial and complete and to fully convey the idea of the present invention to those skilled in the art. Also, in the figures, the thicknesses and sizes of each layer are exaggerated for the convenience of explanation and clarity.

[0035] FIG. 1 shows a separator for a fuel cell according to an embodiment of the present invention.

[0036] 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) containing flake-shaped carbon materials (32) and granular carbon materials (31). The coating layer can be formed by applying, onto the substrate, a composition obtained by mixing 10 to 70 wt% of the binder resin (20) and 30 to 90 wt% of the filler (30) composed of flake-shaped carbon materials and granular carbon materials. As the method for applying the composition, a pre-coating method, a nano-spray method, or the like can be used, but it is not limited thereto.

[0037] The binder resin (20) plays a role of binding the filler (30) made of carbon materials and is an important factor determining the coating physical properties. If the content of the binder resin is higher than 70 wt%, the surface resistance becomes too high, and if it is lower than 10 wt%, the mechanical physical properties of the coating cannot be ensured. Therefore, it is preferably mixed at a ratio of 10 to 70 wt%.

[0038] As the type of the binder resin (20), conventional resins such as Polyacylic, Polyphenolic, and polyester, and conductive resins such as Polyaniline, Polyphenylene surlfide (PPS), and Polyacetylene can be used, but it is not limited thereto. The binder resin composition can be formulated together with various components, such as a solvent, a curing agent, a pigment, a pigment dispersant, and an additive. It may further contain a polymerization initiator such as a thermal initiator and a photoinitiator so that polymerization can be easily carried out.

[0039] The binder resin (20) can be produced through a process of heat curing or photocuring the above-described binder resin composition. For example, the curing reaction can be a solid polymerization using blocked isocyanate. The curing reaction can occur while the blocking agent such as ketone or alcohol dissociates. In the case of the heat curing reaction, the reaction can proceed at 20 to 250 °C. After curing, the conductivity of the binder resin (20) can be in the range of 10 -4 ~10 S / cm.

[0040] The binder resin can be a linear or branched polymer. The more the polymer is branched, the higher the corrosion resistance, but there is a problem that the conductivity becomes lower. Therefore, in the examples of the present invention, a linear polymer can be used and a branched polymer can be appropriately mixed and used. For example, monomers can be grafted onto the linear polymer backbone to introduce branches.

[0041] By uniformly wrapping the filler, the binder resin (20) can adjust the molecular weight and viscosity so that the filler exists in a uniformly dispersed state without sedimentation. Since the dispersibility of the filler is important for determining the conductivity of the separator, it is necessary to uniformly disperse the filler in the binder resin. The viscosity of the mixture increases as the molecular weight of the binder resin increases, so the molecular weight of the binder resin is preferably adjusted to 100 to 5000 MW.

[0042] In the case of an organic paint type coating, although the corrosion resistance is good, there is a problem that the resistance of the resin and the connectivity of the conductive filler are not good and the resistance is high. Here, in the examples of the present invention, after applying a mixture of the binder resin and the filler onto a substrate and then heat curing, the thickness of the coating can be reduced by heat shrinkage so that the filler adheres. The thickness of the coating layer is made 10 μm or less, and an increase in resistance due to an increase in the thickness of the coating can be suppressed.

[0043] To lower the surface resistance of the separator plate, it is important to adjust the type and orientation of the filler (30). The filler (30) is made of carbon materials such as graphene, carbon nanotubes (CNT), graphite, and carbon black, and may further contain metal powder as required. The shape of the filler can affect the formation of a network for electron movement. In one embodiment, highly conductive flaky graphene and carbon nanotubes can be used to form a needle-like or plate-like structure due to their large point and surface contact.

[0044] To prevent the flaky carbon materials from being separated from each other by the binder resin and the conductivity from decreasing, granular graphite or carbon black can be mixed. The granular carbon material can be partially inserted on the surface of the flaky carbon material and play a role in narrowing the distance between the flaky carbon materials, thereby forming a network path for electron movement. At this time, the flaky carbon material and the granular carbon material can be mixed at a weight ratio of 7:3 to 8:2.

[0045] In other embodiments, metal powder can be further added to enhance the conductivity of the filler. The metal powder can be stainless steel or a noble metal series, but is not limited thereto. For example, by mixing 60 - 80 wt% of flaky carbon material, 20 - 30 wt% of granular carbon material, and 0.01 - 10 wt% of metal powder, the conductivity of the filler can be improved.

[0046] To reduce the surface resistance of the separator plate, the flake-shaped carbon material constituting the filler (30) can 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). When the fillers are randomly arranged or oriented at a low angle, the flow of current becomes non-uniform, and the flow of current can be partially interrupted by the polymer resin having insulating properties, increasing the contact resistance and decreasing the electrical conductivity. In one embodiment, after coating the composition containing the filler on the substrate, it may further include a step of orienting the filler at an inclination of 30° or more. For example, during the coating process, a magnetic field or electric field generator can be connected to the coater to generate a magnetic field or electric field around the filler to align the direction of the filler, or a physical force can be applied to the coating layer to orient the filler at 30° or more. Thereby, the flow of electricity in the coating layer can be smoothly controlled.

[0047] In one embodiment, the density of the filler is 10 to 10 4 EA / cm 2 , more preferably 100 to 10 4 EA / cm 2 so as to achieve high conductivity and low contact resistance. The viscosity of the carbon material or the paste in which the carbon material and the metal are mixed can be 10 3 ~10 4 S / cm.

[0048] The separator plate for a fuel cell manufactured according to an embodiment of the present invention exhibits conductivity of 500 S / cm or more and has a contact resistance value of 35 mΩ / cm 2 or less without applying pressure and 5 mΩ / cm 2 or less under the state of applying a pressure of 133 N / cm 2 .

[0049] The amount of the filler to be mixed with the binder resin is desirably 30 to 90 wt%. When the filler content is less than 30 wt%, a network path for electron movement cannot be formed, and it is difficult to expect an improvement in conductivity. When it exceeds 90 wt%, there is a problem that the dispersibility of the filler deteriorates and the moldability decreases.

[0050] Figure 2 is a schematic diagram showing the step of brushing the surface of the coating layer to expose the filler. By removing the binder resin that wraps the filler on the surface of the coating layer, the filler can be exposed to the outside. For example, the number of brushing times can be carried out 300 to 2000 times. Figure 2(a) shows that the filler is exposed on the surface of the coating layer by brushing and the conductivity is improved. Figure 2(b) shows that by removing a part of the coating layer, a part of the binder resin located in the outermost layer is removed and the area of the filler increases. Thereby, inside the coating layer, the filler exists in a state of being dispersed surrounded by the binder resin, and on the surface of the coating layer, the filler exists in a state of being exposed to the outside.

[0051] The metal substrate (10) can be formed of titanium or a titanium alloy. Although titanium (Ti) materials are more expensive than stainless materials, they can exhibit high stack performance. Using the same membrane electrode assembly (MEA), at a current density of 1 A / cm 2 as shown in Figure 3 is the absolute ratio of the performance (Power) of the separator plates using stainless steel and Ti materials. 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 more excellent. Also, while the specific gravity of stainless steel is 7.9, Ti is as low as about 4.6, and when applied with the same thickness, a weight reduction of 40% or more is possible.

[0052] 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.

[0053] The present invention has been described with reference to the embodiments shown in the drawings, which are merely illustrative, and those having ordinary knowledge in the relevant technical field will understand that various modifications and equivalent other embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention should be determined by the technical idea of the appended claims.

Industrial Applicability

[0054] The present invention can be utilized in the fields of fuel cells, metal separator plates for fuel cells, coating of metal separator plates, and coating materials, and can improve the reliability and competitiveness of products.

Explanation of Reference Numerals

[0055] 10: Metal substrate 20: Binder resin 30: Filler 31: Granular carbon material 32: Flaky carbon material

Claims

1. A metal substrate; and A coating layer formed on the metal substrate and composed of a mixture of a binder resin and a filler containing a flake-like carbon material and a granular carbon material; Inside the coating layer, the filler exists in a state of being dispersed and wrapped by the binder resin, On the surface of the coating layer, the filler exists in a state of being exposed to the outside, A separator for a fuel cell.

2. The filler further contains metal powder, The separator for a fuel cell according to Claim 1.

3. The filler is At least one or more selected from the group consisting of graphene, carbon nanotubes, graphite, and carbon black, The separator for a fuel cell according to Claim 1.

4. The thickness of the coating layer is formed to be 0.01 to 10 μm, The separator for a fuel cell according to Claim 1.

5. The density of the filler in the coating layer is 10 to 10 ４ EA / cm ２ is. The separator for a fuel cell according to Claim 1.

6. The filler is arranged at an angle of 30 to 90° with respect to the surface of the substrate, The separator for a fuel cell according to Claim 1.

7. The metal substrate is Formed of titanium or a titanium alloy, The separator for a fuel cell according to Claim 1.

8. The binder resin is a linear or branched polymer, The separator for a fuel cell according to Claim 1.

9. The metal powder is stainless steel powder, The separator for a fuel cell according to Claim 2.

10. The step of mixing 10 to 70 wt% of a binder resin and 30 to 90 wt% of a filler composed of a flake-like carbon material and a granular carbon material; The step of applying the mixture onto a substrate and thermally curing it; and The step of brushing the surface of the coating layer cured on the substrate to expose the filler existing on the surface of the coating layer to the outside; including The flake-like carbon material and the granular carbon material are mixed at a weight ratio of 7:3 to 8:2, A coating method for a separator for a fuel cell.

11. The filler is Further contains metal powder and is mixed with 60 to 80 wt% of a flake-like carbon material, 20 to 30 wt% of a granular carbon material, and 0.01 to 10 wt% of metal powder, The coating method for a separator for a fuel cell according to Claim 10.

12. The filler is at least one or more selected from the group consisting of graphene, carbon nanotubes, graphite, and carbon black. The method for coating a separator for a fuel cell according to claim 10.

13. The substrate is formed of titanium or a titanium alloy. The method for coating a separator for a fuel cell according to claim 10.

14. The binder resin is a linear or branched polymer. The method for coating a separator for a fuel cell according to claim 10.

15. The metal powder is stainless steel powder. The method for coating a separator for a fuel cell according to claim 11.

Images