Stainless steel for fuel cell electrode plate and surface modification method thereof

Abstract

The present application relates to the technical fields of fuel cell technology, more particularly to a stainless steel for fuel cell polar plate and a surface modification method thereof. The element composition of the stainless steel comprises Cr: 20-30%, Ni: 0-20%, Mo: 3-10%, N: 0.05-1.5%, C: 0-0.03%, Si: 0-0.6%, Mn: 0-0.5%, Nb: 0-0.5%, W: 0-0.2%, Sn: 0-1%, Au: 0-0.5%, Ta: 0-0.5%, Al: 0-0.02%, Cu: 0-0.2%, P: 0-0.05%, S: 0-0.05%, Ce: 0-0.1%, and Fe makes up the balance. The surface of the stainless steel is removed from the surface passivation film and impurities and a new passivation film is formed, and the self-corrosion current density can be less than 1 mu A / cm 2 , and meanwhile has excellent electric conductivity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fuel cell technology, especially to the field of IPCH01M8 / 021, and more particularly to a stainless steel for fuel cell polar plate and a surface modification method thereof. BACKGROUND

[0002] In recent years, with the progress of fuel cell technology, new requirements have been put forward for the durability of the stack. The corrosion resistance life of metal plate has increased from 5000h to 10000h or 30000h. Currently, 316L stainless steel is commonly used for metal polar plate. However, the passivation film formed by stainless steel is insufficient in corrosion resistance and electrical conductivity. In the later stage of service in the fuel cell environment, the polar plate material exhibits large-area pitting, sudden increase in contact resistance, and iron ion dissolution, which cannot meet the normal use of the fuel cell in the later stage of service. Therefore, there is an urgent need to develop a metal plate material with long service life, high electrical conductivity, and low cost suitable for large-scale production.

[0003] Currently, the mainstream method for improving the service life and performance of bipolar plates is to prepare a protective coating on the surface of the metal material. This method not only significantly increases the cost of the bipolar plate, but also inevitably results in a poor combination between the coating and the substrate due to the incomplete densification of the coating, which leads to a service life of the bipolar plate with coating being usually less than 10000h, which cannot meet the requirements.

[0004] Patent CN202010552720.4 proposes a composition ratio of high-corrosion-resistant ferritic stainless steel for fuel cells. The corrosion resistance of the stainless steel is improved by adding Cr, Mo, and Sn in combination, which improves the corrosion resistance, but the improvement is limited, and the resistance of the passivation film is also relatively high.

[0005] In the literature "Stainless Steel Composition Optimization for Proton Exchange Membrane Fuel Cell Bipolar Plates", the composition of the stainless steel is refined by a cluster method, and the content of Ni and Mo elements in the stainless steel is changed to improve the electrical conductivity of the stainless steel while maintaining good corrosion resistance of the stainless steel. The optimal composition of the stainless steel is proposed. None of the above references proposes a quantitative corrosion resistance index, and no specific solution is given to solve the contradiction between corrosion resistance and electrical conductivity of the passivation film of the stainless steel. SUMMARY

[0006] The first aspect of the present application provides a stainless steel for fuel cell polar plate, the element composition of the stainless steel includes Cr: 20-30%, Ni: 0-20%, Mo: 3-10%, N: 0.05-1.5%, C: 0-0.03%, Si: 0-0.6%, Mn: 0-0.5%, Nb: 0-0.5%, W: 0-0.2%, Sn: 0-1%, Au: 0-0.5%, Ta: 0-0.5%, Al: 0-0.02%, Cu: 0-0.2%, P: 0-0.05%, S: 0-0.05%, Ce: 0-0.1%, Fe makes up the balance.

[0007] The PREN (pitting resistance equivalent number) of the stainless steel is greater than 30.

[0008] The PREN = 1%Cr + 3.3%Mo + 1.5% (Nb+W) + 5%Ta + 16%N.

[0009] Preferably, the PREN of the stainless steel is 30-70.

[0010] The proportion of C in the stainless steel is not higher than 0.01wt%.

[0011] The element of the stainless steel includes at least one of Nb, W and Ta.

[0012] The contact resistance of the stainless steel is lower than 60mΩ·cm under a contact pressure of 1.5MPa. 2 .

[0013] The self-corrosion current density of the stainless steel is not higher than 40μA / cm under the condition of 80℃, pH: 3, H2SO4, 5ppm fluoride ion. 2 .

[0014] The second aspect of the present application provides a surface modification method of a stainless steel for fuel cell polar plate, including the following steps:

[0015] S1, removing the passivation film and impurities on the surface of the stainless steel;

[0016] S2, detecting the removal effect: using organic solvent to clean the stainless steel again, using X-ray photoelectron spectroscopy

[0017] to detect the removal effect;

[0018] S3, immersing the stainless steel into an acid solution, applying a polarization potential for polarization treatment, generating a new passivation film, and obtaining the surface modified stainless steel.

[0019] The applicant finds that a new passivation film is generated by electrochemical polarization or acidification method, and Fe element on the surface of the stainless steel is removed as much as possible, Cr and Mo elements on the surface of the stainless steel are removed as little as possible, then Cr and Mo elements are oxidized to form oxides as much as possible, and a passivation film rich in chromium oxide and molybdenum oxide and containing trace metals (Au and Ta) is formed on the surface, so that the contact resistance is less than mΩ·cm 2 .

[0020] The applicant finds that adding N element can reduce the activity and diffusion coefficient of Cr element in stainless steel, hinder the nucleation and growth of carbides and intergranular compounds, thereby improving the pitting corrosion resistance, intergranular corrosion resistance and crevice corrosion resistance of the stainless steel. The passivation film generated together can have self-healing property under the working condition of the proton exchange membrane fuel cell. After the passivation film serves for a long time under the working condition of the proton exchange membrane fuel cell, local corrosion of the passivation film will inevitably occur due to the erosion of F - and SO4 2- . The fuel cell cathode is an oxidizing environment, and a new passivation film will be generated in the local corrosion area to prevent further corrosion. The fuel cell anode is a reducing environment, and the N element in the stainless steel may react with H + to generate NH4 + , which increases the anode pH to a certain extent and reduces the corrosion reaction rate. In addition, the N element can make the metal quickly repassivate to inhibit the growth of pitting corrosion.

[0021] The acid pickling process, ion etching process or polishing process can be used in the process of the step S1.

[0022] The acid used in the acid pickling process includes at least one of hydrochloric acid, sulfuric acid and hydrofluoric acid.

[0023] Preferably, the acid used in the acid pickling process includes hydrochloric acid, sulfuric acid and hydrofluoric acid, and the weight ratio of the hydrochloric acid, sulfuric acid and hydrofluoric acid is (7-25):(30-75):(0.5-4).

[0024] The ion etching process includes Ar ion etching.

[0025] The polishing process uses at least one of mechanical polishing and electrochemical polishing.

[0026] The pH of the acid solution in the step S3 is less than 6, and the temperature is 10-90℃.

[0027] Preferably, the pH of the acid solution in the step S3 is 2-6, and the temperature is 20-90℃.

[0028] In step S3, the polarization potential under the standard hydrogen electrode (vs. SHE) is 0–1.6 V.

[0029] The thickness of the new passivation film is less than 5 nm.

[0030] The mass ratio of Cr to Fe in the new passivation film is greater than 1.

[0031] The surface-modified stainless steel exhibits a contact resistance of less than 80 mΩ·cm at a contact pressure of 1.5 MPa. 2 .

[0032] The surface-modified stainless steel exhibits a self-corrosion current density of no higher than 1 μA / cm under conditions of 80°C, pH 3, H2SO4, and 5 ppm fluoride ions. 2 .

[0033] Beneficial effects:

[0034] 1. Adding elements such as Ni, Cr, Sn, Au, and Ta improves the electrical conductivity of stainless steel. Ta and Au, in particular, are used to enhance the electrical conductivity of stainless steel.

[0035] Metallic forms improve conductivity; elements such as Ni, Cr, and Sn enhance conductivity by forming passivation films.

[0036] 2. A carbon content of <0.03% can reduce the formation of large carbide particles, which is beneficial to improving the corrosion resistance of stainless steel.

[0037] 3. Adding nitrogen can reduce the activity and diffusion coefficient of chromium in stainless steel, hindering the nucleation and growth of carbides and intergranular compounds, thereby improving the pitting corrosion resistance, intergranular corrosion resistance and crevice corrosion resistance of stainless steel.

[0038] 4. Adding appropriate amounts of rare earth elements can refine the stainless steel grains, stabilize the oxide passivation film, and further improve corrosion resistance, ensuring that the self-corrosion current density of stainless steel under conditions of 80℃, pH: 3, H2SO4, and 5ppm fluoride ions does not exceed 40μA / cm. 2 .

[0039] 5. By performing surface modification on stainless steel using specific methods, a dense, corrosion-resistant, and highly conductive passivation film can be formed on its surface, and the passivation film has self-healing properties under the operating conditions of proton exchange membrane fuel cells.

[0040] 6. The self-corrosion of the surface-modified stainless steel under conditions of 80℃, pH: 3, H2SO4, and 5ppm fluoride ions.

[0041] Corrosion current density not higher than 1 μA / cm 2 . Detailed Implementation

[0042] Examples 1-7, Comparative Example 1

[0043] A stainless steel for fuel cell polar plate, the elemental composition of the stainless steel is shown in Table 1 in percentage by weight.

[0044] Table 1

[0045]

[0046] The other elements include Mn, Al, Cu, Si, P, S, usually a variety of small amounts, and not fixed, and it is difficult to detect accurately by prior art, and no effect on product performance, so no specific content of other elements is limited.

[0047] The stainless steel preparation method is the following steps: smelting, forging, hot working, cold working in turn, and the like.

[0048] Examples 8-14

[0049] A surface modification method of a stainless steel for fuel cell polar plate, the method is the following steps:

[0050] S1, 7 pieces of stainless steel prepared in Example 7 are cut into a shape with a length and width of 5 cm and a thickness of 1 cm, the stainless steel is cleaned using mechanical polishing to remove the surface passivation film, and the pretreated stainless steel is obtained;

[0051] S2, the stainless steel is immersed in acetone again for ultrasonic treatment for 600 s, and the composition of the surface of the stainless steel is tested using X-ray photoelectron spectroscopy

[0052] The composition, the surface passivation film and impurities are confirmed to be removed;

[0053] S3, the stainless steel is immersed in a sulfuric acid solution respectively, the pH is adjusted as shown in Table 2, and a polarization potential as shown in Table 2 is applied respectively for polarization treatment for 1200 s, and the surface modified stainless steel is obtained.

[0054] Table 2

[0055] Example pH of sulfuric acid solution Temperature of sulfuric acid solution / °C Polarization potential / V (vs. SHE) Example 8 2 20 0 Example 9 4 20 0 Example 10 6 20 0 Example 11 2 55 0 Example 12 2 90 0 Example 13 2 55 0.8 Example 14 2 55 1.6

[0056] Performance test method

[0057] 1. PREN (pitting resistance equivalent) in Comparative Example 1 and Examples 1-7 is calculated by the elemental composition of the stainless steel, PREN = 1% Cr + 3.3% Mo + 1.5% (Nb + W) + 5% Ta + 16% N, wherein the comparative example is 316L stainless steel. The test results are shown in Table 3.

[0058] 2. The phase structure of the stainless steel produced in Comparative Example 1 and Examples 1-7 was observed by a microscope. The test results are shown in Table 3.

[0059] 3. The electric conductivity and corrosion resistance of the stainless steel produced in Comparative Example 1 and Examples 1-7 were measured, wherein the contact resistance (mΩ-cm 2 ) was tested at a contact pressure of 1.5 MPa; and the self-corrosion current density (μA / cm 2 ) was tested at 80°C, pH: 3, H2SO4, 5 ppm fluoride ion. The test results are shown in Table 3.

[0060] 4. The electric conductivity and corrosion resistance of the surface-modified stainless steel of Examples 8-14 were tested, wherein the contact resistance (mΩ-cm 2 ) was tested at a contact pressure of 1.5 MPa; and the self-corrosion current density (μA / cm 2 ) was tested at 80°C, pH: 3, H2SO4, 5 ppm fluoride ion. The test results are shown in Table 4.

[0061] 5. The content of Fe ion and Cr ion (mg / L) in the sulfuric acid solution of the surface-modified stainless steel of Examples 8-14 was tested. The test results are shown in Table 4.

[0062] Performance test data and conclusions

[0063] Table 3

[0064]

[0065]

[0066] Without any treatment, the initial contact resistance of the comparative example was 87 mΩ-cm 2 in terms of electric conductivity, the resistance was the largest; the contact resistance of the stainless steel of Examples 1-4 was in the range of 42-55 mΩ-cm 2 , the resistance was relatively high, the contact resistance of the stainless steel of Examples 5-7 was in the range of 22-27 mΩ-cm 2 , the resistance was relatively small, but still greater than 10 mΩ-cm 2 . The reason for the lower contact resistance of Examples 5-7 was that Sn and Au elements which could improve the electric conductivity were added to the stainless steel.

[0067] In terms of corrosion resistance, the self-corrosion current density of the comparative example was 57 μA / cm 2 , the corrosion current density was the largest; the self-corrosion current density of Example 1 was 39 μA / cm 2 , the corrosion current density was relatively large; the self-corrosion current density of Examples 2, 6 was in the range of 5-18 μA / cm 2, the corrosion current density is smaller; in Example 7, the self-corrosion current density is in the range of 0.4 μA / cm 2 , the corrosion current density is the smallest.

[0068] From the above test data, the conductive performance and corrosion resistance of Comparative Example 1 are poor, the conductive performance and corrosion resistance of the examples are improved to a certain extent compared with Comparative Example 1, the corrosion resistance is improved more significantly, but due to the high contact resistance, it cannot be directly used for fuel cells.

[0069] Table 4

[0070] Example Fe ion content Cr ion content Contact resistance Self-corrosion current density Example 8 26.46 0.05 5.5 0.25 Example 9 17.56 0.04 6.2 0.31 Example 10 3.94 0.06 5.8 0.78 Example 11 38.78 0.57 4.6 0.41 Example 12 20.72 0.38 13.7 0.38 Example 13 45.80 0.62 3.2 0.16 Example 14 70.55 0.74 2.5 0.07

[0071] In Example 14, the Fe ion precipitation is the most, the Cr ion precipitation changes less, and the ion concentration changes less under different conditions. In terms of electrical conductivity, the contact resistance of other conditions is less than 10 mΩ·cm 2 , the self-corrosion current density is less than 1 μA / cm 2 , the self-corrosion current density of Example 14 is smaller, meeting the requirements of the U.S. Department of Energy on the conductive and corrosion performance of the plate material. Therefore, the performance of the stainless steel after treatment in Example 14 is the best.

Claims

1. A stainless steel electrode plate for a fuel cell, characterized in that, The stainless steel is composed of the following elements by weight percentage: Cr: 26%, Ni: 5%, Mo: 8%, N: 0.5%, C: 0.008%, Nb: 0.3%, Sn: 0.5%, Au: 0.3%, Ta: 0.2%, other elements 2.0%, and Fe to make up the balance; the other elements include Mn, Al, Cu, Si, P, and S. The stainless steel has a contact resistance of 24.1 mΩ·cm at a contact pressure of 1.5 MPa. 2 ; The self-corrosion current density of the stainless steel under the conditions of 80℃, pH: 3, H2SO4, and 5ppm fluoride ions is 0.4μA / cm. 2 ; The surface modification method for stainless steel is characterized by comprising the following steps: S1, removes the passivation film and impurities from the stainless steel surface; S2, Testing the removal effect: The stainless steel was cleaned again with an organic solvent, and the removal effect was tested using X-ray photoelectron spectroscopy. S3, stainless steel is immersed in an acid solution and polarized by applying a polarization potential to generate a new passivation film, thus obtaining surface-modified stainless steel. In step S3, the pH of the acid solution is <6 and the temperature is 10-90℃; the polarization potential under the standard hydrogen electrode in step S3 is 0~1.6V.

Images