MnCo2O4 / Co3O4 coating for solid oxide fuel cell connector and preparation method of MnCo2O4 / Co3O4 coating

Abstract

The invention discloses a MnCo2O4 / Co3O4 coating for a solid oxide fuel cell (SOFC) connector and a preparation method of the MnCo2O4 / Co3O4 coating, and belongs to the technical field of SOFC key component coatings. The coating is of a double-layer oxide skin structure, the inner layer is a Cr2O3 layer, the outer layer is a MnCo2O4 / Co3O4 composite layer, and the coating is prepared on the surface of an SUS 430 steel base material through a two-step electrodeposition and high-temperature oxidation process. The preparation method comprises the following four steps: pre-treating the base material; carrying out electro-deposition on the MnO2 nanosheet layer in the first step; carrying out electro-deposition on the Co layer in the second step; and carrying out high-temperature oxidation conversion in an air atmosphere of 800 DEG C. After the prepared coating is oxidized at 800 DEG C for 5 weeks, the surface specific resistance (ASR) is 4.97 m omega.cm < 2 >, the oxidation parabola rate constant kp is 0.0306 mg < 2 >. Cm <-4 >. Week <-1 >, and the coating has excellent high-temperature oxidation resistance, high conductivity and Cr volatilization inhibition capacity; and the preparation process is simple and controllable, low in cost and suitable for large-scale production, and can be widely applied to performance improvement of the medium-temperature SOFC connector.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of solid oxide fuel cell (SOFC) key component coating technology, and particularly relates to a MnCo2O4 / Co3O4 coating for a SOFC interconnect and a preparation method thereof. BACKGROUND

[0002] As a kind of high-efficiency and clean energy conversion device, the interconnect of solid oxide fuel cell (SOFC) is one of the core components, which plays a key role in transmitting heat and electric energy, and isolating adjacent single cell anode and cathode side fuel gas and air. With the working temperature of SOFC decreasing from 1000℃ to 600-800℃, ferritic stainless steel (such as SUS 430) gradually replaces ceramic material to become the preferred material of the interconnect due to its good thermal conductivity, mechanical stability and cost advantage.

[0003] However, ferritic stainless steel still faces two major problems during the operation of SOFC: first, the surface is prone to form an oxide skin, which leads to an increase in contact resistance and affects the efficiency of cell electric energy transmission; second, Cr element is prone to volatilize and migrate to the cathode, causing cathode poisoning, which seriously reduces the durability and service life of the cell. Therefore, developing a high-performance protective coating is the key to solving the above problems.

[0004] Mn-Co spinel oxide is considered to be an ideal interconnect protective coating material due to its low oxygen diffusion coefficient and strong Cr absorption capacity. In the prior art, researchers use methods such as electrochemical deposition and electrophoretic deposition to prepare Mn-Co-based coatings, but there are problems such as uneven distribution of coating elements, insufficient adhesion to the substrate, and poor high-temperature stability. For example, it is difficult to achieve uniform distribution of Mn and Co elements by using a single electro-deposition method, which leads to poor oxidation resistance and electrical performance of the coating; and the coating prepared by traditional high-temperature sintering method is prone to cracking and cannot effectively inhibit Cr volatilization. Therefore, it is of great practical application value to develop a preparation method with simple process and excellent coating performance. SUMMARY

[0005] (I) Invention purpose The present application aims to provide a MnCo2O4 / Co3O4 coating for a solid oxide fuel cell interconnect and a preparation method thereof. By using a two-step electro-deposition combined with a high-temperature oxidation process, a protective coating with a double-layer structure is obtained, which solves the problems of poor high-temperature oxidation resistance, Cr volatilization leading to cathode poisoning, and excessive surface resistance of the existing SOFC interconnect.

[0006] (II) Technical solutions To achieve the above-mentioned purpose, the present application adopts the following technical solutions: 1. Coating structure design: the coating is a double-layer oxide skin structure, the inner layer is a dense Cr2O3 layer that can block the outward diffusion of metal ions in the substrate; the outer layer is a MnCo2O4 / Co3O4 composite layer that has a low oxygen diffusion coefficient and high electrical conductivity, can inhibit the inward diffusion of oxygen, reduce the coating resistance, and effectively absorb Cr elements to prevent their volatilization; 2. Preparation method: (1) Substrate pretreatment: polishing and cleaning the SUS 430 steel substrate to remove surface impurities and oxide skin and improve the adhesion of the coating to the substrate; (2) Two-step electrodeposition: in the first step, MnO2 nanosheet layers are deposited in a specific electrolyte to provide a uniform template for subsequent Co deposition; in the second step, a Co layer is deposited in a Co salt electrolyte to fill the gaps between the MnO2 nanosheets and form a dense Co-MnO2 composite coating; (3) High-temperature oxidation conversion: air oxidation is carried out at the working temperature of SOFC (800℃) to convert the Co-MnO2 composite coating into a MnCo2O4 / Co3O4 spinel composite layer and form a stable Cr2O3 inner layer.

[0007] (Three) Beneficial effects The beneficial effects of the present application are as follows: 1. Excellent coating performance: after 800℃, 5-week oxidation, the area specific resistance of the coated stainless steel is only 4.97mΩ·cm 2 , which is much lower than that of the substrate SUS 430 steel (35.86mΩ·cm 2 ); the oxidation parabolic rate constant is 0.0306mg 2 ·cm -4 ·week -1 , which is significantly lower than that of the substrate itself (0.0811mg 2 ·cm -4 ·week -1 ), indicating that the coating has excellent high-temperature oxidation resistance and electrical properties; 2. Effective inhibition of Cr volatilization: the outer MnCo2O4 / Co3O4 composite layer has strong adsorption capacity for Cr, which can significantly inhibit the outward diffusion of Cr and prevent the poisoning of the SOFC cathode; 3. Simple and controllable preparation process: the two-step electrodeposition process requires low equipment and is simple to operate, and can achieve uniform deposition of the coating; the high-temperature oxidation conditions match the working temperature of SOFC, and no additional high-temperature treatment is required, reducing production costs; 4. Strong adhesion: the MnO2 nanosheets form a stable bond with the substrate, and the Co layer fills the gaps to form a dense structure, and after high-temperature oxidation, the coating is firmly bonded to the substrate without falling off. BRIEF DESCRIPTION OF DRAWINGS

[0008] Figure 1 XRD patterns and surface morphology of Co-MnO2 composite coating; wherein (a) is the XRD pattern of Co-MnO2 coating after two-step electrodeposition, (b) is the surface morphology of MnO2 coating after the first step of electrodeposition, and (c) is the surface morphology of Co-MnO2 coating after the second step of electrodeposition; Figure 2 Parabolic oxidation kinetics curves of uncoated steel and Co-MnO2 coated steel in air atmosphere at 800℃; Figure 3 Cross-sectional morphology, EDS line scanning pattern and XRD pattern of Co-MnO2 coated steel after oxidation for different times; wherein (a) is the cross-sectional morphology and EDS line scanning after 1 week of oxidation, (b) is the cross-sectional morphology and EDS line scanning after 3 weeks of oxidation, (c) is the cross-sectional morphology and EDS line scanning after 5 weeks of oxidation, and (d) is the XRD pattern under different oxidation times. DETAILED DESCRIPTION

[0009] Example 1: Preparation of MnCo2O4 / Co3O4 coating 1. Substrate pretreatment: select 10mm x 10mm x 1mm SUS 430 steel sheet, polish the surface smooth with 800# and 1200# SiC sandpaper in turn, then ultrasonic clean in acetone and deionized water for 15min respectively to remove surface oil and impurities, and finally dry at 60℃ in nitrogen atmosphere for 30min for standby; 2. First step of electrodeposition (MnO2 layer preparation): build a three-electrode system with the treated SUS 430 steel sheet as the cathode, a graphite rod as the anode, and a saturated calomel electrode as the reference electrode; the electrolyte is composed of 0.1M Mn(OAc)2·4H2O and 0.1M Na2SO4, the electrodeposition temperature is controlled at 25℃, the current density is 10mA·cm -2 , the deposition time is 16min, and MnO2 nanosheet coating is obtained; 3. Second step of electrodeposition (Co layer preparation): immediately immerse the steel sheet with MnO2 coating into an electrolyte with pH of 4, which contains 40g·L -1 CoSO4·7H2O, 10g·L-1CoCl2·6H2O, 15g·L -1 H3BO3 and 0.2g·L -1 sodium dodecylbenzenesulfonate (surfactant); keep the cathode and anode unchanged, control the electrodeposition temperature at 50℃, the current density at 15mA·cm -2 , and the deposition time at 8min to obtain Co-MnO2 composite coating; 4. High-temperature oxidation conversion: The Co-MnO2 composite coating steel sheet was hung in an alumina crucible and placed in a muffle furnace for oxidation treatment in a static air atmosphere at 800°C for 5 weeks. During the period, the sample was taken out every week, cooled to room temperature, and then the mass was measured with an electronic balance with a precision of 0.01 mg. Finally, the MnCo2O4 / Co3O4 double-layer structure coating was obtained.

[0010] Example 2: Coating performance test 1. Phase analysis: The phase of the coating was analyzed by X-ray diffractometer (XRD, X'Pert PRO, PANalytical), and the results are shown in Figure 1 (a). After two-step electrodeposition, the coating contains characteristic diffraction peaks of Co, MnO2 and α-Fe (substrate), indicating that Co and MnO2 are successfully co-deposited; 2. Microstructure analysis: The surface and cross-sectional morphology of the coating were observed by scanning electron microscope (SEM, S-4800, Hitachi High-Technol.), and the results are shown in Figure 1 (b), (c). After the first step of electrodeposition, vertically arranged MnO2 nanosheets (thickness ~ 20 nm, length ~ 500 nm) are formed. After the second step of electrodeposition, Co fills the gap between the nanosheets, forming a dense composite coating. After 5 weeks of oxidation, the coating forms a double-layer structure, with a 2 μm thick Cr2O3 layer as the inner layer and a 5 μm thick MnCo2O4 / Co3O4 layer as the outer layer Figure 3 (c); 3. Oxidation performance test: The high-temperature oxidation resistance of the coating was evaluated by mass gain method, and the results are shown in Figure 2 . The mass gain of the coated steel is significantly lower than that of the uncoated steel, and the parabolic rate constant k p = 0.0306 mg 2 ·cm -4 ·week -1 , which is much lower than the 0.0811 mg 2 ·cm -4 ·week -1 of the uncoated steel; 4. Electrical performance test: The area specific resistance (ASR) of the coating was tested by four-point direct current method at 600-800°C (interval 50°C). After 5 weeks of oxidation, the ASR value of the coated steel is 4.97 mΩ·cm 2 , which is significantly lower than the 35.86 mΩ·cm 2 of the uncoated steel.

[0011] Example 3: Cr evaporation inhibition performance test of the coating The coated steel and uncoated steel prepared in Example 1 were respectively oxidized in air at 800°C for 5 weeks. The Cr element distribution in the coating was analyzed by EDS line scanning. Figure 3 (c)) The results showed that the Cr content in the outer layer of the coating was only 2 at.%, indicating that the MnCo2O4 / Co3O4 outer layer effectively blocked the outward diffusion of Cr and could avoid SOFC cathode poisoning.

[0012] Supplementary explanation of specific implementation methods 1. In this invention, the current density for the first electrodeposition step can be 8–12 mA·cm⁻¹. -2 The deposition time can be adjusted within a certain range. Uniform MnO2 nanosheets can be obtained by adjusting the time within the range of 12 to 20 minutes. 2. The surfactant used in the second electrodeposition step can be replaced with other anionic surfactants (such as sodium dodecyl sulfate) without affecting the deposition effect of the Co layer; 3. The high-temperature oxidation time can be adjusted according to actual needs. After 3 weeks of oxidation, a basically stable MnCo2O4 / Co3 can be formed. The O4 coating reaches its optimal performance after 5 weeks of oxidation.

Claims

1. A MnCo2O4 / Co3O4 coating for a solid oxide fuel cell interconnect, characterized by: The coating is a double-layer oxide skin structure, the inner layer is a Cr2O3 layer, and the outer layer is a MnCo2O4 / Co3O4 composite layer; the coating is prepared on the surface of a SUS 430 steel substrate through two-step electrodeposition combined with a high-temperature oxidation process, and after oxidation at 800 DEG C in an air atmosphere for 5 weeks, the area specific resistance (ASR) of the coating is 4.97 m Omega cm 2 , the oxidation kinetics parabolic rate constant k p is 0.0306 mg 2 cm -4 week -1 .

2. A method of preparing the MnCo204 / Co304 coating layer according to claim 1, characterized in that, The method comprises the following steps: (1) substrate pretreatment: the SUS 430 steel substrate is sequentially polished, ultrasonically cleaned and dried; (2) first step of electrodeposition (MnO2 layer preparation): electrodeposition is performed in an electrolyte with a pretreated SUS 430 steel as a cathode and a graphite rod as an anode; the electrolyte is composed of 0.1M Mn(OAc)2·4H2O and 0.1M Na2SO4, the electrodeposition temperature is 25℃, and the time is 16min; (3) Second step electrodeposition (Co layer preparation): The sample treated in step (2) is immersed in an electrolyte with a pH of 4, and electrodeposition is continued using SUS 430 steel as the cathode and a graphite rod as the anode; the electrolyte contains 40 g·L -1 CoSO4·7H2O, 10g·L -1 CoCl2·6H2O, 15g·L -1 H3BO3 and 0.2 g·L -1 Surfactant, electrodeposition temperature 50℃, current density 15 mA·cm -2 The time is 8 minutes; (4) high-temperature oxidation conversion: the Co-MnO2 composite coating sample obtained in step (3) is placed in an alumina crucible and oxidized at 800℃ in an air atmosphere for 5 weeks to obtain a MnCo2O4 / Co3O4 double-layer structure coating.

3. The method of claim 2, wherein: In step (1), the polishing is sequentially performed with 800# and 1200# SiC sandpaper; the ultrasonic cleaning is sequentially performed in acetone and deionized water, and each cleaning time is 15min; and the drying is performed at 60℃ in a nitrogen atmosphere for 30min.

4. The method of claim 2, wherein: In step (2), the electrodeposition process is carried out in galvanostatic mode at a current density of 10 mA-cm -2 .

5. The method of claim 2, wherein: In step (3), the surfactant is sodium dodecyl benzene sulfonate.

6. The method of claim 2, wherein: In step (4), a static air atmosphere is adopted in the high-temperature oxidation process, and the sample is taken out every week during the oxidation process, cooled to room temperature and subjected to quality detection.

7. Use of the MnCo204 / Co304 coating according to claim 1 in the interconnect of a solid oxide fuel cell (SOFC), characterized in that: The working temperature of the SOFC is 600-800℃, and the coating is used for improving the high-temperature oxidation resistance of the connecting body, inhibiting Cr volatilization and reducing the surface oxide skin resistance.

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