Austenitic stainless steel for polymer fuel cell separators with improved contact resistance and method of manufacturing the same

Abstract

The present specification discloses austenitic stainless steels for fuel cell separators with improved contact resistance. One embodiment of the austenitic stainless steels for fuel cell separators with improved contact resistance disclosed herein can include, by weight percent: up to 0.1% C (excluding 0); up to 3.0% Si (excluding 0); up to 3.0% Mn (excluding 0); 20% to 30% Cr; 8% to 20% Ni; up to 0.003% S; up to 0.03% P; up to 0.6% Mo (excluding 0); up to 0.8% Cu (excluding 0); 0.1% to 0.3% N; and up to 2.0% W (excluding 0), with the remainder being Fe and other unavoidable impurities.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an austenitic stainless steel for a polymer fuel cell separator and a method of manufacturing the same, and more particularly, to a stainless steel for a polymer fuel cell separator having improved contact resistance and a method of manufacturing the same. BACKGROUND

[0002] A polymer electrolyte fuel cell is a fuel cell that uses a polymer membrane having proton exchange properties as an electrolyte. The polymer electrolyte fuel cell has advantages of low operating temperature, high current density, high output density, fast startup, and fast response to load time changes compared to other types of fuel cells.

[0003] The polymer electrolyte fuel cell includes unit cells each made of a membrane electrode assembly (MEA) including an electrolyte, an electrode, and a gas diffusion layer (GDL), and a separator. A structure composed of a plurality of unit cells connected in series is referred to as a fuel cell stack.

[0004] The separator, which is a core component of the polymer electrolyte fuel cell stack, is an electrically conductive plate provided with a gas flow passage of an oxidation electrode (or a fuel electrode) on one side and a gas flow passage of a reduction electrode (or an air electrode) on the other side.

[0005] The separator acts as a current collector that conducts electrons generated at the oxidation electrode toward the reduction electrode of the next cell and supports the MEA. In addition, the separator acts as a passage for removing water generated when the fuel cell is operated while fuel (hydrogen or reforming gas) and oxidant (oxygen and air) are supplied to the electrodes of the fuel cell, respectively.

[0006] Graphite having a flow passage formed by mechanical processing has been conventionally used for most separators. However, graphite is not suitable for mass production due to processing difficulty and high price. For these reasons, stainless steel has been widely used in recent years in consideration of manufacturing cost and weight. In the case of using stainless steel as a separator of a polymer electrolyte fuel cell, a stainless steel plate having a thickness of 0.1 mm is generally used.

[0007] Since coil tension control is difficult and in order to prevent surface defects such as indentation defects formed after cold rolling, the stainless steel plate is not annealed in an oxidizing atmosphere but is bright annealed in a reducing atmosphere using hydrogen or nitrogen for recrystallization and removal of residual stress. Since an oxide film formed by bright annealing has high resistance, in order to use stainless steel as a separator of a fuel cell, a post-processing step for improving interface contact resistance is required.

[0008] As a post-treatment step, a method of coating stainless steel with a conductive material such as gold (Au), carbon, or nitride has been proposed. However, such a method can cause problems of an increase in manufacturing cost and an increase in manufacturing time due to the additional process of coating the noble metal.

[0009] (Prior Art Documents)

[0010] (Patent Document 1) Korean Patent Laid-Open No. 10-2010-0073407 (July 1, 2010) SUMMARY

[0011] TECHNICAL PROBLEM

[0012] To solve the above problems, the present disclosure provides a stainless steel for a polymer fuel cell separator having an improved interfacial contact resistance without additional surface treatment such as coating on an austenitic stainless steel only by AC electrolysis for a short time, and a manufacturing method of the same.

[0013] TECHNICAL SOLUTION

[0014] According to one aspect of the present disclosure, an austenitic stainless steel for a fuel cell separator having an improved contact resistance includes, by weight percent (wt.%): up to 0.1% of C (excluding 0), up to 3.0% of Si (excluding 0), up to 3.0% of Mn (excluding 0), 20% to 30% of Cr, 8% to 20% of Ni, up to 0.003% of S, up to 0.03% of P, up to 0.6% of Mo (excluding 0), up to 0.8% of Cu (excluding 0), 0.1% to 0.3% of N, up to 2.0% of W (excluding 0), and the remainder being Fe and other inevitable impurities.

[0015] Further, in the present disclosure, the austenitic stainless steel can include 0.01% to 0.5% of W by weight percent (wt.%).

[0016] Further, in the present disclosure, the austenitic stainless steel can have an interfacial contact resistance of up to 10 mΩ·cm 2 (100 N / cm 2 ).

[0017] According to another aspect of the present disclosure, a method of manufacturing an austenitic stainless steel for a fuel cell separator having improved contact resistance includes: bright annealing a cold-rolled austenitic stainless steel containing, in weight percent (wt.%): up to 0.1% of C (excluding 0), up to 3.0% of Si (excluding 0), up to 3.0% of Mn (excluding 0), 20% to 30% of Cr, 8% to 20% of Ni, up to 0.003% of S, up to 0.03% of P, up to 0.6% of Mo (excluding 0), up to 0.8% of Cu (excluding 0), 0.1% to 0.3% of N, up to 2.0% of W (excluding 0), and the remainder being Fe and other inevitable impurities; and performing an alternating current electrolysis on the bright annealed material in a sulfuric acid solution, wherein the alternating current electrolysis is performed by applying a current density of 15 A / dm 2 to 30 A / dm 2 for 7 seconds to 10 seconds.

[0018] Further, in the present disclosure, the austenitic stainless steel can contain 0.01% to 0.5% of W in weight percent (wt.%).

[0019] Further, in the present disclosure, the bright annealing can be performed at a temperature of 1050°C to 1150°C.

[0020] Further, in the present disclosure, the temperature of the sulfuric acid solution can be 40°C to 80°C.

[0021] Further, in the present disclosure, the concentration of the sulfuric acid solution can be 50 g / L to 300 g / L.

[0022] Further, in the present disclosure, the frequency of the alternating current can be 10 Hz to 120 Hz.

[0023] Advantageous Effects

[0024] According to one embodiment of the present disclosure, a stainless steel for a polymer fuel cell separator having improved contact resistance by optimizing electrolysis conditions and a method of manufacturing the same are provided. DETAILED DESCRIPTION

[0025] According to one embodiment of the present disclosure, a stainless steel for a polymer fuel cell separator having improved contact resistance by optimizing electrolysis conditions and a method of manufacturing the same can be provided.

[0026] Embodiment of the Invention

[0027] Preferred embodiments of this disclosure will now be described below. However, this disclosure may be implemented in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be detailed and complete, and will fully communicate the scope of this disclosure to those skilled in the art.

[0028] The terminology used herein is for the purpose of describing embodiments only. Therefore, unless the context clearly distinguishes them, the singular usage encompasses the plural usage. Furthermore, it should be understood that terms such as “comprising” or “having” are intended to indicate the presence of features, steps, functions, components, or combinations thereof disclosed in the specification, and are not intended to exclude the possibility that one or more other features, steps, functions, components, or combinations thereof may be present or added.

[0029] Furthermore, unless otherwise specified, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. Therefore, these terms should not be interpreted in an idealized or overly formal sense unless expressly stated otherwise herein. Unless the context clearly indicates otherwise, the singular form as used herein is intended to include the plural form as well.

[0030] Furthermore, the terms “about,” “substantially,” etc., used throughout this specification mean, when stating the permissible error of natural manufacture and substances, that such permissible error corresponds to a value or similar value, and that such value is intended for clear understanding of this disclosure or to prevent unintentional infringers from unlawfully using the contents of this disclosure.

[0031] According to one embodiment of this disclosure, the austenitic stainless steel for fuel cell separators with improved contact resistance comprises, by weight percentage (wt%): up to 0.1% C (excluding O), up to 3.0% Si (excluding O), up to 3.0% Mn (excluding O), 20% to 30% Cr, 8% to 20% Ni, up to 0.003% S, up to 0.03% P, up to 0.6% Mo (excluding O), up to 0.8% Cu (excluding O), 0.1% to 0.3% N, up to 2.0% W (excluding O), and the remainder being Fe and other unavoidable impurities. The reasons for the numerical limitations on the content of alloying elements in the embodiments of this disclosure will be described below.

[0032] The content of C is at most 0.1% (excluding 0).

[0033] Carbon (C), as an inexpensive austenite stabilizing element, effectively suppresses the formation of the delta (δ) ferrite phase. Furthermore, C is an interstitial element and improves the yield strength of steel through solid solution strengthening. However, since excessive C degrades the ductility, toughness, and corrosion resistance of steel, its upper limit is controlled to 0.1%. Therefore, the C content can be controlled to a maximum of 0.1% (excluding 0).

[0034] The Si content is at most 3.0% (excluding 0).

[0035] Silicon (Si) is an element added during steelmaking as a deoxidizer. When added in a certain amount, it improves corrosion resistance by forming Si-oxides in the passivation layer through bright annealing. However, excessive Si includes the formation of intermetallic compounds such as δ-ferrite and σ phases during casting, leading to a deterioration in the hot workability, ductility, and toughness of the steel. Therefore, the Si content can be controlled to a maximum of 3.0% (excluding 0).

[0036] The Mn content is at most 3.0% (excluding 0).

[0037] Manganese (Mn) acts as an austenite phase stabilizer, effectively inhibiting martensite formation. Furthermore, Mn is cheaper than Ni and improves the cold workability of steel. However, excessive Mn leads to the formation of numerous inclusions (MnS), which degrades the hot workability, ductility, and toughness of the steel. Therefore, the Mn content should be controlled to a maximum of 3.0% (excluding 0%).

[0038] The Cr content is 20% to 30%.

[0039] Chromium is added to improve corrosion resistance by forming a passivation layer in oxidizing environments, and it is added in amounts of 20% or more to achieve corrosion resistance in fuel cell environments. However, when the Cr content exceeds 30%, it promotes the formation of delta (δ) ferrite in the slab, leading to a deterioration in the hot workability of the steel. Furthermore, because austenite becomes unstable, a large amount of Ni is required to achieve phase stability, resulting in increased costs. Therefore, the Cr content can be controlled at 20% to 30%.

[0040] The Ni content is 8% to 20%.

[0041] Nickel (Ni) acts as an austenite phase stabilizing element, suppressing the formation of the delta (δ)-ferrite phase, and its addition at 8% or more improves hot and cold workability. However, as a high-priced element, the addition of Ni in large quantities leads to increased raw material costs, therefore its upper limit is controlled at 20%.

[0042] The content of P is at most 0.03%, and the content of S is at most 0.003%.

[0043] Because phosphorus (P) and sulfur (S) are harmful elements that segregate at grain boundaries, deteriorating corrosion resistance and hot workability, the contents of P and S should be controlled to be as low as possible. Therefore, the P content can be controlled to a maximum of 0.03%, and the S content can be controlled to a maximum of 0.003%.

[0044] The content of Mo is at most 0.6% (excluding 0).

[0045] Molybdenum (Mo) is an element added to stainless steel to improve its corrosion resistance. However, as a high-priced element, adding large amounts of Mo increases raw material costs and deteriorates cold workability. Therefore, the Mo content should be controlled to a maximum of 0.6% (excluding 0).

[0046] The Cu content is at most 0.8% (excluding 0).

[0047] Copper (Cu), as an austenite phase stabilizing element, effectively improves cold workability and corrosion resistance of steel in reducing environments by inhibiting martensite formation. However, excessive Cu can degrade hot workability due to solidification segregation. Therefore, the Cu content should be controlled to a maximum of 0.8% (excluding 0).

[0048] The nitrogen content is 0.1% to 0.3%.

[0049] Nitrogen (N) has the effect of stabilizing the austenitic phase and improving the strength of materials, so nitrogen is added in an amount of 0.1% or greater. However, since excessive N may degrade elongation, its upper limit is controlled at 0.3%. Therefore, the N content can be controlled in the range of 0.1% to 0.3%.

[0050] The content of W is at most 2.0% (excluding 0).

[0051] Tungsten (W) improves corrosion resistance and reduces interfacial contact resistance in sulfuric acid atmospheres during fuel cell operation. In particular, corrosion resistance can be maximized by simultaneously adding W and Cu in an environment where sulfuric acid is concentrated. However, excessive W may lead to increased raw material costs (because W is a high-priced element) and deterioration of elongation. Therefore, the W content can preferably be controlled to a maximum of 2.0% (excluding 0). A more preferred lower limit for W content is 0.01%, and a more preferred upper limit is 0.5%.

[0052] The remaining component of the composition of this disclosure is iron (Fe). However, the composition may contain unintended impurities inevitably introduced from raw materials or the surrounding environment. The addition of other unintended alloying elements besides those described above is not excluded in this disclosure. Impurities are not specifically mentioned in this disclosure because they are known to those skilled in the art.

[0053] Because the separator serves as the electrical pathway for electrons generated and consumed at the electrodes during fuel cell operation, excellent conductivity is required between the separator and the gas diffusion layer. In other words, the resistance of the separator significantly affects fuel cell performance; therefore, the interfacial contact resistance of the materials used for the separator needs to be reduced to below permissible levels.

[0054] Based on the functional requirements of the separator, the U.S. Department of Energy (DOE) recommends that the target interfacial contact resistance of the separator should be 10 mΩ·cm. 2 (100N / cm 2 Up to 150 N / cm 2 (or smaller.)

[0055] According to one embodiment of this disclosure, the austenitic stainless steel can have a strength of 10 mΩ·cm. 2 (100N / cm 2 (or smaller interface contact resistance).

[0056] According to one embodiment of this disclosure, austenitic stainless steel is manufactured by the following method.

[0057] The method includes: bright annealing cold-rolled austenitic stainless steel, the cold-rolled austenitic stainless steel comprising, by weight percentage (wt%): up to 0.1% C (excluding O), up to 3.0% Si (excluding O), up to 3.0% Mn (excluding O), 20% to 30% Cr, 8% to 20% Ni, up to 0.003% S, up to 0.03% P, up to 0.6% Mo (excluding O), up to 0.8% Cu (excluding O), 0.1% to 0.3% N, up to 2.0% W (excluding O), and the remainder being Fe and other unavoidable impurities; and alternating current electrolysis of the bright-annealed material in a sulfuric acid solution, wherein the alternating current electrolysis is performed by applying 15 A / dm 2 Up to 30A / dm 2 The current density is applied for 7 to 10 seconds.

[0058] Furthermore, according to one embodiment of this disclosure, austenitic stainless steel may also contain 0.01% to 0.5% W by weight percentage (wt%).

[0059] The reasons for imposing numerical limits on the content of alloying elements are as described above.

[0060] Furthermore, according to one embodiment of this disclosure, bright annealing can be carried out at a temperature of 1050°C to 1150°C.

[0061] Bright annealing refers to annealing performed in a non-oxidizing atmosphere. Typically, coils with a thickness of 0.3 mm or less are bright annealed in a reducing atmosphere containing hydrogen and nitrogen due to the difficulty in controlling coil tension and to prevent surface defects. In this case, the hydrogen content can be 70% or more. Furthermore, the bright annealing temperature can be from 1050°C to 1150°C to suppress re-oxidation of cold-rolled austenitic stainless steel during the heat treatment process.

[0062] Since bright annealing is carried out in a reducing atmosphere, a passivation layer with a smooth surface and a thickness of several nanometers can be formed. This passivation layer may contain Cr-Fe oxides, Mn oxides, Si oxides, etc.

[0063] Cold-rolled and bright-annealed steel may have increased contact resistance due to a passivation layer formed on its surface that is several nanometers thick. Therefore, in order to use cold-rolled and bright-annealed steel as a fuel cell separator, it is necessary to remove the non-conductive passivation layer formed on the surface and form a new conductive layer thereon.

[0064] Meanwhile, the flow of electricity, or electric current, is broadly classified into two types: direct current (DC) and alternating current (AC). Alternating current refers to an electric current that periodically reverses direction and continuously changes its magnitude over time. Electrolysis is a technique that decomposes substances by inducing chemical changes as an electric current passes through an electrolyte; it is also known as electrolytic decomposition. Electrolysis is classified into DC electrolysis and AC electrolysis according to the type of power source. In AC electrolysis, the electrode acts as the positive electrode at one moment and then as the negative electrode at the next moment, allowing oxidation and reduction to occur sequentially on a single electrode.

[0065] When austenitic stainless steel surfaces are improved using DC electrolysis, its high interfacial contact resistance makes it difficult to use as a material for fuel cell separators. As a result of considering various control conditions to address this issue, AC power was introduced.

[0066] Therefore, according to one embodiment of the present disclosure, a passivation layer is removed from cold-rolled and bright-annealed austenitic stainless steel by alternating electrolysis in a sulfuric acid solution, and a conductive film with improved interfacial contact resistance is formed thereon.

[0067] In this disclosure, all types of waveforms, such as sine waves, square waves, triangle waves, and sawtooth waves, can be applied to AC power supplies.

[0068] Meanwhile, when the applied current density is less than 15 A / dm 2 The film formed by bright annealing is difficult to remove. When excessive current density is applied, the passivation layer removal effect saturates, potentially leading to side reactions such as oxygen generation or surface corrosion due to over-acid pickling. Therefore, the applied current density should be controlled at 15 A / dm³. 2 Up to 30A / dm 2 Within the range.

[0069] Furthermore, according to one embodiment of this disclosure, the frequency of the applied AC can be from 10 Hz to 120 Hz.

[0070] When the frequency of the applied AC is less than 10Hz, the efficiency improvement decreases, so the frequency can be controlled in the range of 10Hz to 120Hz.

[0071] Furthermore, according to one embodiment of this disclosure, AC electrolysis can be performed for 7 to 10 seconds. When the AC electrolysis time is less than 7 seconds, the pickling effect of hot-rolled and annealed steel sheets cannot be obtained. When the AC electrolysis time is greater than 10 seconds, operational efficiency cannot be achieved.

[0072] Furthermore, according to one embodiment of this disclosure, the temperature of the sulfuric acid solution can be from 40°C to 80°C.

[0073] When the temperature of the sulfuric acid solution is below 40℃, the passivation layer removal efficiency decreases. Considering safety, the upper limit of the temperature can be controlled at 80℃.

[0074] Furthermore, according to one embodiment of this disclosure, the concentration of the sulfuric acid solution can be from 50 g / L to 300 g / L.

[0075] When the concentration of sulfuric acid solution is less than 50 g / L, the removal of the passivation layer may be insufficient due to the decrease in the conductivity of the solution. Conversely, even when the concentration of sulfuric acid is significantly increased, the passivation layer removal effect is saturated. Therefore, considering the economic feasibility of electrolysis, the concentration of sulfuric acid solution can be controlled at 300 g / L or less.

[0076] The present disclosure will be described in more detail below through embodiments. However, it should be noted that the following embodiments are intended only to illustrate the present disclosure in more detail and are not intended to limit the scope of the present disclosure. This is because the scope of the present disclosure is determined by the matters described in the claims and that can be reasonably inferred from them.

[0077] Example

[0078] Slabs with the alloying elements shown in Table 1, prepared by continuous casting, were heated at 1,250°C for 2 hours and hot-rolled, followed by hot annealing at 1,100°C for 90 seconds. The resulting material was then cold-rolled with a reduction of 70%, and bright annealed at 1,050°C after cold rolling.

[0079] Table 1

[0080] Steel grade C Si Mn Cr Ni Mo Cu N W Steel grade A 0.025 0.4 0.8 21.3 10.5 0.6 0.8 0.2 0.01 Steel grade B 0.02 0.2 3 22 11 0.1 0.1 0.15 0.01 Steel grade C 0.03 2 0.5 22 12.5 0.1 0.1 0.2 0.5

[0081] Subsequently, electrolysis was performed under the conditions shown in Table 2 below, and the interfacial contact resistance values ​​under these conditions were measured. The evaluation of the interfacial contact resistance was carried out as follows. Two plates, each with a 50cm diameter, were prepared. 2 Prepare materials for the area, using a piece of material with a diameter of 4cm. 2 A carbon paper (SGL-10BA) is inserted between the layers and used as a gas diffusion layer, then at 100 N / cm². 2 The interfacial contact resistance was evaluated 5 times under the contact pressure.

[0082] Table 2

[0083]

[0084] Referring to Table 2, under the sulfuric acid and current conditions specified in this disclosure, electrolysis can yield up to 10 mΩ·cm. 2 Interface contact resistance.

[0085] Conversely, in Comparative Example 1, the surface improvement effect decreased with a frequency of 5 Hz, thus achieving 25.5 mΩ·cm. 2 Slightly higher interfacial contact resistance.

[0086] In Comparative Example 2, DC electrolysis was performed instead of AC electrolysis, thus obtaining 52.5 mΩ·cm. 2 High interfacial contact resistance.

[0087] In Comparative Examples 3 and 4, an application of less than 15 A / dm was used. 2 The current density was so high that the layer formed by bright annealing was not removed, resulting in a current density exceeding 10 mΩ·cm. 2 The high interfacial contact resistance. In Comparative Example 5, because the temperature of the sulfuric acid solution was below 40°C, the removal of the layer formed by bright annealing was insufficient, resulting in a resistance of 31.7 mΩ·cm. 2 High interfacial contact resistance.

[0088] According to the disclosed embodiments, without additional surface treatments such as coating, the contact resistance can be 10 mΩ·cm by optimizing the current density and frequency conditions during the electrolysis process. 2Or smaller, therefore, austenitic stainless steel according to this disclosure can be used as a material for polymer fuel cell separators.

[0089] Although this disclosure has been specifically described with reference to exemplary embodiments, those skilled in the art will understand that various changes in form and detail may be made without departing from the spirit and scope of this disclosure.

[0090] Industrial applicability

[0091] The austenitic stainless steel for fuel cell separators according to this disclosure has improved contact resistance through optimized electrolysis conditions, thus eliminating the need for additional post-processing steps and enabling its industrial use by reducing manufacturing costs and time.

Claims

1. A method of manufacturing an austenitic stainless steel for fuel cell separators having improved contact resistance, the method comprising: bright annealing a cold-rolled austenitic stainless steel containing, in weight percent (wt%), more than 0% and at most 0.1% of C, more than 0% and at most 3.0% of Si, more than 0% and at most 3.0% of Mn, 20% to 30% of Cr, 8% to 20% of Ni, at most 0.003% of S, at most 0.03% of P, more than 0% and at most 0.6% of Mo, more than 0% and at most 0.8% of Cu, 0.1% to 0.3% of N, more than 0% and at most 2.0% of W, and the remainder being Fe and other unavoidable impurities; and subjecting the bright annealed material to an alternating current electrolysis in a sulfuric acid solution at a temperature of 40°C to 80°C, wherein a conductive film is formed on the surface by the alternating current electrolysis. wherein the alternating current electrolysis is performed at a frequency of 10 Hz to 120 Hz by applying a current density of 15 A / dm 2 to 30 A / dm 2 for 7 seconds to 10 seconds.

2. The method according to claim 1, wherein the austenitic stainless steel contains 0.01% to 0.5% of W in weight percent (wt%).

3. The method according to claim 1, wherein the bright annealing is performed at a temperature of 1050°C to 1150°C.

4. The method according to claim 1, wherein the concentration of the sulfuric acid solution is 50 g / L to 300 g / L.

5. An austenitic stainless steel for fuel cell separators having improved contact resistance, obtained by the method according to claim 1, containing, in weight percent (wt%), more than 0% and at most 0.1% of C, more than 0% and at most 3.0% of Si, more than 0% and at most 3.0% of Mn, 20% to 30% of Cr, 8% to 20% of Ni, at most 0.003% of S, at most 0.03% of P, more than 0% and at most 0.6% of Mo, more than 0% and at most 0.8% of Cu, 0.1% to 0.3% of N, more than 0% and at most 2.0% of W, and the remainder being Fe and other unavoidable impurities, and the austenitic stainless steel includes a conductive film.

6. The austenitic stainless steel according to claim 5, wherein the austenitic stainless steel contains 0.01% to 0.5% of W in weight percent (wt%). wherein the interfacial contact resistance of the austenitic stainless steel is at most 10 mΩ-cm 2 . ​

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