Steel material for polymer fuel cell seperator and method for manufacturing same

Abstract

The present invention relates to a steel material for a polymer fuel cell separator and a method for manufacturing same. The steel material for a polymer fuel cell separator comprises: a steel material from which an oxide film is removed; and a carbon coating layer formed on the surface of the steel material, wherein the carbon coating layer contains 90 wt % or more of graphite having a SP2 bond, and has excellent conductivity and oxidation resistance.

Description

Steel for polymer fuel cell separator and manufacturing method thereof

[0001] The present invention relates to a steel material for a polymer fuel cell separator and a method for manufacturing the same.

[0002] Currently, austenitic stainless steels, such as 316L and 304, are primarily used as separator materials for polymer fuel cells. This is due to the excellent high-temperature oxidation resistance of the oxide film formed on the surface. However, scale formed in a polymer fuel cell environment must possess not only excellent oxidation resistance but also excellent contact conductivity. The oxide film formed on the surface of the steel plate can be damaged by peeling, which can reduce conductivity and lower fuel cell efficiency.

[0003] The oxide film that typically forms on the surface of stainless steel is primarily composed of Cr2O3, which provides it with oxidation resistance. However, while Cr2O3 offers excellent oxidation resistance, it also has low electrical conductivity, which can be a weakness when using stainless steel as a separator material.

[0004] In addition to Cr2O3, Al and Si can easily form oxides at high temperatures, and although Al and Si oxides have excellent corrosion resistance, they reduce electrical conductivity, so the formation of Al and Si oxides should be suppressed as much as possible. However, since Al and Si are mainly used during the deoxidation process to remove oxygen in the steelmaking process of stainless steel, it is difficult to completely remove Al and Si from stainless steel. To solve this problem, a method of adding rare earth elements has been proposed, but it is difficult to apply it to the general stainless steel manufacturing process, and there is a problem that the manufacturing cost increases excessively.

[0005] Therefore, to solve these problems, it is necessary to develop stainless steel with excellent conductivity and oxidation resistance.

[0006] One aspect of the present invention is to provide a steel material for a polymer fuel cell separator and a method for manufacturing the same.

[0007] A preferred aspect of the present invention is to provide a steel material for a polymer fuel cell separator having excellent conductivity and oxidation resistance and a method for manufacturing the same.

[0008] One embodiment of the present invention provides a steel for a polymer fuel cell separator, comprising: a steel material from which an oxide film has been removed; and a carbon coating layer formed on the surface of the steel material; wherein the carbon coating layer contains 90 wt% or more of graphite having an SP2 bond.

[0009] The above steel may be stainless steel.

[0010] The above steel contains C: 0.02% or less (excluding 0%), N: 0.02% or less (excluding 0%), Si: 0.4% or less (excluding 0%), Mn: 0.2% or less (excluding 0%), Cr: 25-32%, Cu: 0.2% or less (excluding 0%), Ni: 0.8% or less (excluding 0%), Ti: 0.5% or less (excluding 0%), Nb: 0.5% or less (excluding 0%), and the remainder may be composed of Fe and other unavoidable impurities.

[0011] The average thickness of the above carbon coating layer may be 50 to 200 nm.

[0012] The above steel may have a contact resistance of 20 mΩ㎠ or less and a surface resistance of 6 mΩ㎠ or less.

[0013] Another embodiment of the present invention provides a method for manufacturing a steel for a polymer fuel cell separator, comprising the steps of: preparing a steel material having an oxide film formed thereon; etching the surface of the steel material to a thickness of 30 nm or more to remove the oxide film from the steel material; and forming a carbon coating layer on the surface of the steel material from which the oxide film has been removed using a physical vapor deposition method under conditions where the surface temperature of the steel material is 50° C. or lower and a vacuum atmosphere is used.

[0014] The step of removing the above oxide film is performed under the conditions of a plasma ion source pressure: 1.5 to 2.1 mTorr, Ar flow rate: 8 to 11 sccm in a vacuum atmosphere at 86 to 164 W / 100 cm 2 It can be authorized with the strength of .

[0015] The application of the above plasma ion source can be performed under the condition of a current of 43 to 82 mA at a voltage of 2 kV.

[0016] According to one aspect of the present invention, a steel material for a polymer fuel cell separator and a method for manufacturing the same can be provided.

[0017] According to a preferred aspect of the present invention, a steel material for a polymer fuel cell separator having excellent conductivity and oxidation resistance and a method for manufacturing the same can be provided.

[0018] Hereinafter, a steel material for a polymer fuel cell separator according to one embodiment of the present invention will be described.

[0019] A steel material for a polymer fuel cell separator according to one embodiment of the present invention includes: a steel material from which an oxide film has been removed; and a carbon coating layer formed on the surface of the steel material.

[0020] The present invention is not particularly limited to the applicable steel material, but may be, for example, stainless steel. The stainless steel may be ferritic stainless steel. The stainless steel has a property of easily forming an oxide film due to a large amount of elements such as chromium, and the oxide film has the characteristic of excellent oxidation resistance but low electrical conductivity. Therefore, the present invention is characterized by removing the oxide film from the steel material. Through this, excellent conductivity can be secured. In addition, the adhesion between the steel material and the carbon coating layer formed on the surface of the steel material can be improved. Meanwhile, the present invention is not particularly limited to the alloy composition of the steel, but may include C: 0.02% or less (excluding 0%), N: 0.02% or less (excluding 0%), Si: 0.4% or less (excluding 0%), Mn: 0.2% or less (excluding 0%), Cr: 25 to 32%, Cu: 0.2% or less (excluding 0%), Ni: 0.8% or less (excluding 0%), Ti: 0.5% or less (excluding 0%), Nb: 0.5% or less (excluding 0%), and the remainder may be made of Fe and other unavoidable impurities.

[0021] The above carbon coating layer is generally known to have excellent oxidation resistance. However, in the present invention, it is preferable that the carbon coating layer contains more than 90 wt% of graphite having an SP2 bond. The graphite may generally have an SP2 bond or an SP3 bond. However, graphite having an SP3 bond has a disadvantage of low conductivity. Therefore, in the present invention, by containing more than 90 wt% of graphite having an SP2 bond, not only oxidation resistance but also excellent conductivity can be secured. If the fraction of graphite having an SP2 bond in the carbon coating layer is less than 90 wt%, contact resistance and surface resistance are low, making it difficult to secure excellent conductivity.

[0022] The average thickness of the carbon coating layer may be 50 to 200 nm. If the average thickness of the carbon coating layer is less than 50 nm, sufficient adhesion may not be secured, which may result in the carbon coating layer peeling off or cracks occurring in the surface coating layer during forming and processing of the steel material. If the average thickness exceeds 200 nm, the manufacturing cost may increase excessively.

[0023] As described above, the steel according to one embodiment of the present invention may have a contact resistance of 20 mΩ㎠ or less and a surface resistance of 6 mΩ㎠ or less, and thus may be preferably used as a polymer fuel cell separator.

[0024] Hereinafter, a method for manufacturing a steel material for a polymer fuel cell separator according to one embodiment of the present invention will be described.

[0025] First, a steel material with an oxide film formed is prepared. The present invention does not specifically limit the steel material preparation process, and a steel material with an oxide film that hinders improved conductivity, such as stainless steel, can be prepared.

[0026] Thereafter, the surface of the steel is etched to a thickness of 30 nm or more to remove the oxide film from the steel. The oxide film formed on the steel typically has a thickness of 5 to 30 nm, and in the present invention, it is preferable to perform etching to a thickness of 30 nm or more so that the oxide film is completely removed. If the etching thickness is less than 30 nm, sufficient adhesion between the steel and the carbon coating layer may not be secured, resulting in peeling. Meanwhile, the present invention does not specifically limit the upper limit of the etching thickness, but for example, the upper limit may be 50 nm in consideration of the uneven etching effect due to surface roughness.

[0027] At this time, in the step of removing the oxide film, the plasma ion source is used in a vacuum atmosphere at a pressure of 1.5 to 2.1 mTorr, and an Ar flow rate of 8 to 11 sccm, at 86 to 164 W / 100 cm 2It can be applied with the intensity of . This sets the condition for stable plasma discharge, and if the condition is not satisfied, plasma discharge may not occur, making it difficult to remove the oxide film. Meanwhile, in the present invention, the voltage and current conditions for the application intensity of the plasma ion source are not particularly limited, but the application can be performed under the condition of current of 43 to 82 mA at a voltage of 2 kV.

[0028] Thereafter, a carbon coating layer is formed on the surface of the steel from which the oxide film has been removed using a physical vapor deposition method under conditions where the surface temperature of the steel is 50°C or lower and a vacuum atmosphere is used. If the surface temperature of the steel exceeds 50°C, not only may an oxide film be formed on the surface of the steel, but also a large amount of graphite having an SP3 bond rather than an SP2 bond may be formed, making it difficult to secure excellent conductivity. Therefore, the surface temperature of the steel is preferably 50°C or lower. It is more advantageous for the surface temperature of the steel to be 30°C or lower.

[0029] Hereinafter, the present invention will be described in more detail through examples. However, the following examples are merely illustrative examples for further explaining the present invention and do not limit the scope of the present invention.

[0030] (Example 1)

[0031] In order to find conditions under which plasma discharge can be stably performed, a remote ion source etching process was performed while changing the pressure, Ar flow rate, and applied intensity as shown in Table 1 below.

[0032] Condition pressure (mTorr)Ar flow rate (sccm)Applied intensity (W / 100cm 2)Voltage (Kv)Current (mA)Discharge presence or absence10.85522211None21.0630215None31.3742221None41.5863243Yes51.79106253Yes61.910134267Yes72.111164282Yes82.31244222None93.82020210None

[0033] As can be seen from the above Table 1, the pressure suggested by the present invention: 1.5 to 2.1 mTorr, Ar flow rate: 8 to 11 sccm, and applied intensity: 86 to 164 W / 100 cm 2 It can be confirmed that stable discharge is taking place under these conditions.

[0034] (Example 2)

[0035] For stainless steel material containing C: 0.02%, N: 0.02%, Si: 0.4%, Mn: 0.2%, Cr: 30%, Cu: 0.2%, Ni: 0.8%, Ti: 0.5%, Nb: 0.5% in weight%, and the remainder being Fe and other unavoidable impurities, pressure: 1.8 mTorr, Ar flow rate: 10 sccm, and applied intensity: 105.8~114.0 W / 100 cm 2 The oxide film was removed by etching at a thickness as described in Table 2 below under the conditions of (voltage: 2 kV, current: 64 to 69 mA). Thereafter, a carbon coating layer was formed on the surface of the stainless steel material from which the oxide film was removed using the conditions described in Table 2 below. The graphite fraction having SP2 bonds, contact resistance, surface resistance, and adhesion of the steel material thus manufactured were measured, and the results are shown in Table 2 below.

[0036] The fraction of graphite with SP2 bonds was measured using the relative ratio of the absorption intensities of the D band and the G band measured by Raman scattering spectroscopy.

[0037] The contact resistance was measured by cross-stacking three GDL (Gas Diffusion Layer) and two separator specimens on a Cu plate, and then stacking a Cu plate on top of them, and applying a resistance of 100 N / cm to the Cu plate side. 2 The resistance value (mohm) measured under pressure was measured and the GDL area was multiplied.

[0038] The surface resistance was measured as an average value using a 4-point probe on the surface of the manufactured steel material.

[0039] Adhesion was evaluated by attaching 3M tape to steel, performing 0t bending, peeling the tape, and then peeling the tape off to the surface where the tape was attached. A coating peeling of 0% was judged as good, and a coating peeling of more than 0% was judged as poor.

[0040] Classification Etching thickness (nm) Steel surface temperature (℃) Graphite fraction (wt%) with SP2 bonding Carbon coating layer thickness (nm) Contact resistance (mΩ㎠) Surface resistance (mΩ㎠) Adhesion Invention example 130 25 90 100 16.85 310 Good Comparative example 15 30 075 100 55.65 426 Bad Comparative example 21 5 30 060 100 238.65 413 Bad Comparative example 33 0 30 050 100 5 17.06.231 Good

[0041] As can be seen from Table 2 above, in the case of Invention Example 1, which satisfies the conditions of the present invention, it can be seen that excellent conductivity and adhesion are secured.

[0042] On the other hand, in the case of comparative examples 1 to 3 that do not meet the conditions of the present invention, it can be seen that the conductivity or adhesion is poor.

Claims

1. Steel from which the oxide film has been removed; and Including a carbon coating layer formed on the surface of the steel; The above carbon coating layer is a steel for a polymer fuel cell separator containing 90 wt% or more of graphite having an SP2 bond.

2. In claim 1, The above steel is stainless steel for polymer fuel cell separator plates.

3. In claim 1, The above steel contains C: 0.02% or less (excluding 0%), N: 0.02% or less (excluding 0%), Si: 0.4% or less (excluding 0%), Mn: 0.2% or less (excluding 0%), Cr: 25 to 32%, Cu: 0.2% or less (excluding 0%), Ni: 0.8% or less (excluding 0%), Ti: 0.5% or less (excluding 0%), Nb: 0.5% or less (excluding 0%), and the remainder is Fe and other unavoidable impurities.

4. In claim 1, Steel for polymer fuel cell separator having an average thickness of the carbon coating layer of 50 to 200 nm.

5. In claim 1, The above steel is a steel for a polymer fuel cell separator having a contact resistance of 20 mΩ㎠ or less and a surface resistance of 6 mΩ㎠ or less.

6. Step for preparing steel on which an oxide film has been formed; A step of etching the surface of the steel material to a thickness of 30 nm or more to remove an oxide film from the steel material; and A method for manufacturing a steel for a polymer fuel cell separator, comprising: a step of forming a carbon coating layer on the surface of a steel material from which the oxide film has been removed using a physical vapor deposition method under conditions of a surface temperature of 50°C or lower and a vacuum atmosphere.

7. In claim 6, The step of removing the above oxide film is performed under the conditions of a plasma ion source pressure: 1.5 to 2.1 mTorr, Ar flow rate: 8 to 11 sccm, and 86 to 164 W / 100 cm in a vacuum atmosphere. 2A method for manufacturing a steel material for a polymer fuel cell separator having a strength of .

8. In claim 7, A method for manufacturing steel for a polymer fuel cell separator, wherein application of the above plasma ion source is performed under conditions of a voltage of 2 kV and a current of 43 to 82 mA.