Preparation method of Ce modified Mn-Co-O spinel composite coating

Abstract

The application relates to a preparation method of a Ce modified Mn-Co-O spinel composite coating and belongs to the technical field of stainless steel connecting body protection of solid oxide fuel cells. The application adopts a multi-stage cathode micro-arc electrodeposition mode to deposit a Mn-Co-Ce precursor coating on the surface of a stainless steel base body; under the action of a complexing agent, the Ce element exists in a solid solution + grain boundary segregation mode and forms a Mn-Co structure and a structure in which Ce doping and defects coexist in the precursor coating; the precursor coating is in-situ converted into the Ce modified Mn-Co-O spinel composite coating through homogenization heat treatment in a protective atmosphere and oxidation treatment in an air atmosphere; the Ce element is partially solid-solved in a Mn-Co-O crystal lattice, and the rest of the Ce element is dispersedly distributed in a grain boundary region in the form of CeO2 nanoparticles; a composite barrier layer with a synergistic structure of solid solution strengthening-grain boundary pinning-diffusion blocking is constructed; the composite barrier layer can effectively inhibit the outward diffusion of Cr ions and the inward penetration of oxygen ions, significantly reduce the growth rate of a thermally grown oxide layer, and improve the compactness and long-term conductive stability of the coating.

Description

Technical Field

[0001] This invention relates to a method for preparing a Ce-modified Mn-Co-O spinel composite coating, belonging to the field of stainless steel connector protection technology for solid oxide fuel cells. Background Technology

[0002] The connector is a key component in an SOFC stack, primarily serving to achieve electrical connection, gas isolation, and mechanical support between individual cells. Currently, commercially available SOFC connectors mainly use ferritic stainless steel materials, such as AISI 430 and Crofer 22 APU. These materials offer advantages such as low cost, good thermal expansion coefficient matching, and ease of processing. However, under long-term service conditions of 700–900℃, they are prone to high-temperature oxidation, resulting in the formation of a continuously thickening Cr2O3 oxide layer and a Cr-rich spinel layer on the surface, leading to a continuous increase in interfacial contact resistance (ASR). Simultaneously, Cr diffuses and volatilizes at high temperatures, generating Cr-containing volatiles that migrate to the cell cathode, causing a decrease in cathode catalytic activity—a phenomenon known as "Cr poisoning"—which severely affects the output performance and lifespan of the SOFC stack.

[0003] To improve the high-temperature stability of interconnects, researchers have proposed constructing conductive spinel protective coatings on the surface of metal interconnects. Among these, Mn-Co spinel (MnCo2O4) has become one of the most promising interconnect protection materials due to its excellent electronic conductivity, thermal stability, and thermal expansion coefficient matching the substrate. Mn-Co spinel coatings not only reduce interfacial contact resistance but also, to some extent, hinder the outward diffusion of Cr, thus attracting widespread attention. Currently, such coatings can be prepared using methods such as plasma spraying, screen printing, sol-gel, electrodeposition, and micro-arc deposition. However, while existing Mn-Co spinel protective coatings can reduce the oxidation rate and interfacial contact resistance of interconnects to some extent, they still have significant shortcomings. First, traditional Mn-Co coatings are prone to grain coarsening and grain boundary migration under long-term high-temperature environments, leading to the formation of pores and microcracks within the coating, reducing structural density. Second, the deposited coating contains a large number of non-equilibrium grain boundaries and defect structures, which can lead to Cr diffusion. 3+ Fe 3+ Metal cations and O 2- Ions can diffuse rapidly along grain boundaries, forming a thermally grown oxide layer (TGO), leading to Cr depletion in the substrate, interfacial oxidation, and protective layer failure. Furthermore, with prolonged oxidation, metallurgical interdiffusion easily occurs at the coating-substrate interface, resulting in a continuous thickening of Cr-rich oxides, causing a sustained increase in ASR (Acrylic Stain Reduction) and affecting the long-term conductivity of the battery. In addition, existing Mn-Co coatings have limited ability to suppress Cr volatilization, making it difficult to fundamentally solve the Cr poisoning problem during SOFC operation.

[0004] Meanwhile, traditional preparation processes such as screen printing, plasma spraying, and sol-gel methods generally suffer from problems such as high coating porosity, poor thickness uniformity, insufficient bonding strength, and difficulty in controlling composition, making it difficult to obtain functional coatings that combine high density and high stability. Therefore, there is an urgent need to develop a rare earth-modified Mn-Co composite coating and its preparation method that can construct a stable diffusion barrier layer, effectively inhibit Cr and O diffusion, reduce the growth rate of TGO, and improve long-term conductivity stability. Summary of the Invention

[0005] To address the shortcomings of existing methods in preparing spinel MnCo2O4 and its modified coatings, which struggle to achieve both high density and high stability, this invention proposes a method for preparing a Ce-modified Mn-Co-O spinel composite coating. This method involves depositing a Mn-Co-Ce precursor coating on a stainless steel substrate using a multi-stage cathode micro-arc electrodeposition process. Ce, under the influence of a complexing agent, exists synergistically through solid solution and grain boundary segregation, forming a structure in the precursor coating where Mn-Co structures and Ce doping and defects coexist. After maintenance... Homogenization heat treatment under a protective atmosphere and in-situ oxidation treatment under an air atmosphere are used to transform Ce-modified Mn-Co-O spinel composite coatings. Ce elements are partially dissolved in the Mn-Co-O lattice, while the remaining Ce elements are dispersed in the grain boundary region in the form of CeO2 nanoparticles, constructing a composite barrier layer with a synergistic structure of "solid solution strengthening-grain boundary pinning-diffusion blocking". This can effectively inhibit the outward diffusion of Cr ions and the inward penetration of oxygen ions, significantly reduce the growth rate of thermally grown oxide layers, and improve the density and long-term electrical conductivity stability of the coating.

[0006] A method for preparing a Ce-modified Mn-Co-O spinel composite coating, the specific steps of which are as follows: (1) The ferritic stainless steel connector substrate is successively ground, degreased and cleaned, and pickled and activated to obtain a surface-pretreated ferritic stainless steel connector substrate; (2) Using a surface-pretreated ferritic stainless steel connector substrate as the cathode and graphite as the anode, a multi-stage cathode micro-arc electrodeposition was carried out in a manganese-cobalt-cerium composite electrolyte to form a Mn-Co-Ce precursor coating. (3) The ferritic stainless steel connector with deposited Mn-Co-Ce precursor coating is placed in a protective atmosphere for homogenization heat treatment, and then placed in an air atmosphere for oxidation treatment so that the Mn-Co-Ce precursor coating is transformed in situ into Ce modified Mn-Co-O spinel composite coating. The Ce element is partially dissolved in the Mn-Co-O lattice, and the remaining Ce element is dispersed in the grain boundary region in the form of CeO2 nanoparticles to construct a composite barrier layer that inhibits the external diffusion of Cr and the internal diffusion of O.

[0007] Preferably, the acid in step (1) is dilute hydrochloric acid or dilute sulfuric acid, and the concentration of the acid solution is 1~5wt%.

[0008] Preferably, the acid washing activation temperature in step (1) is 10~40℃ and the time is 1~10min.

[0009] Preferably, the manganese-cobalt-cerium composite electrolyte in step (2) contains 20-50 g / L manganese sulfate, 5-20 g / L cobalt sulfate, 0.05-5 g / L cerium sulfate, and 40-130 mg / L complexing agent, and the solvent is deionized water; the complexing agent is one or more of hydroxyethylidene diphosphonic acid, sodium citrate, and trisodium phosphate.

[0010] Preferably, the specific method for the multi-stage cathode micro-arc electrodeposition in step (2) is as follows: 1) One-stage constant current cathode micro-arc electrodeposition: DC current density of 5~10 A / dm 2 The time is 1 to 5 minutes; 2) Two-stage pulsed current cathode micro-arc electrodeposition: peak current density is 1.0~3.0 A / dm² 2 The positive and negative pulse frequencies are 300~1000Hz, the duty cycle is 30%~80%, and the duration is 20~60 min.

[0011] Preferably, the specific method of the homogenization heat treatment in step (3) is as follows: the temperature is uniformly increased to 700-900℃ at a heating rate of 1-5℃ / min and held for 4-10h, and then uniformly increased to 1000-1100℃ at a heating rate of 10-20℃ / min and held for 2-10min.

[0012] Preferably, the specific method for the oxidation treatment in step (3) is as follows: cool down to 700~900℃, then introduce air and keep warm for 2~6 hours.

[0013] The mechanism by which Ce element regulates the structure and properties of the Mn-Co-O spinel composite coating in this invention is as follows: During the multi-stage cathode micro-arc electrodeposition process, Ce element exists in a synergistic manner of solid solution and grain boundary segregation under the action of a complexing agent, forming a structure in the precursor coating where Mn-Co structure and Ce doping and defects coexist. Homogenization heat treatment in a protective atmosphere ensures uniform distribution of Mn, Co, and Ce elements in the coating; rapid heating for a short time accelerates Ce diffusion, forming Ce-rich grain boundary regions; subsequent oxidation treatment in an air atmosphere transforms the precursor coating in situ to form the Mn-Co-Ce-O spinel composite coating; wherein, Ce element is partially dissolved in the Mn-Co-O lattice, and the remaining Ce element is dispersed in the grain boundary region as CeO2 nanoparticles, thereby constructing a composite barrier layer that inhibits the outward diffusion of Cr and the inward diffusion of O.

[0014] The Ce-rich grain boundary structure of this composite coating produces significant "solute dragging effect" and "grain boundary pinning effect," effectively inhibiting abnormal spinel grain growth and grain boundary migration, and maintaining the long-term dense structure of the coating. Simultaneously, the Ce segregation layer can block Cr... 3+ Diffusion along grain boundaries and O 2- The inward-penetrating short-circuit diffusion channels transform the diffusion mechanism from rapid grain boundary diffusion to a slower lattice diffusion, fundamentally reducing the formation rate of the thermally grown oxide layer (TGO). Furthermore, the dense and stable Mn-Co-O structure and CeO2 grain boundary network provide continuous small polariton hopping channels for electron transport, maintaining a low areal resistivity while suppressing oxidation. This forms a synergistic mechanism of "Ce segregation blocking diffusion channels - suppressing TGO growth - maintaining the spinel conductive network," achieving a simultaneous improvement in high-temperature oxidation resistance and conductivity.

[0015] The beneficial effects of this invention are: (1) This invention achieves densification and structural optimization of Ce-modified Mn-Co-O spinel composite coatings by precisely controlling the electrolyte concentration, pulse voltage, deposition time, and duty cycle during the multi-stage cathode micro-arc electrodeposition process, as well as the homogenization and in-situ oxidation heat treatment temperature and time. After cathode micro-arc electrodeposition and high-temperature in-situ conversion, the coating forms a composite structure consisting of Mn-Co-O spinel as the continuous framework phase, Ce solid solution strengthening phase, and CeO2 nano-dispersed phase. Among them, a small amount of Ce ions enter the spinel lattice to form a solid solution, causing local lattice distortion and improving the stability of the crystal structure; most of Ce preferentially segregates to the grain boundaries due to its large ionic radius and is uniformly dispersed in the grain boundary region in the form of nano-CeO2 particles. This structure can not only suppress the abnormal coarsening of spinel grains under high temperature environment through the "grain boundary pinning effect", but also form a continuous diffusion barrier network at the grain boundaries to effectively block Cr. 3+ outward diffusion and O 2- A rapid channel for inward infiltration; (2) Unlike traditional Mn-Co coatings that rely solely on spinel phase to block diffusion, this invention achieves multi-level synergistic regulation of structure through “lattice solid solution strengthening, grain boundary dispersion pinning, and diffusion channel blocking”, which fundamentally improves the high-temperature stability, oxidation resistance, and conductivity of the coating. Attached Figure Description

[0016] Figure 1 The surface morphology of the Ce-modified Mn-Co-O spinel composite coating in Example 1; Figure 2 The XRD results of the Ce-modified Mn-Co-O spinel composite coating in Example 1 are shown. Figure 3 The cross-sectional morphology of the Ce-modified Mn-Co-O spinel composite coating in Example 2 is shown. Figure 4 Analysis of Ce occurrence states in Ce-modified Mn-Co-O spinel composite coating for Example 2; Figure 5 Analysis of Ce valence state in Ce-modified Mn-Co-O spinel composite coating for Example 3; Figure 6 Example 3: Comparison of weight gain during high-temperature oxidation of Ce-modified Mn-Co-O spinel composite coating; Figure 7 Example 3: High-temperature ASR comparison of Ce-modified Mn-Co-O spinel composite coating. Detailed Implementation

[0017] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the content described.

[0018] Example 1: A method for preparing a Ce-modified Mn-Co-O spinel composite coating, the specific steps of which are as follows: (1) The ferritic stainless steel connector substrate is subjected to mechanical grinding, degreasing and cleaning, and pickling and activation (at a temperature of 40℃, with a concentration of 1wt% dilute hydrochloric acid for 1 min) to obtain a surface-pretreated ferritic stainless steel connector substrate. (2) Using a surface-pretreated ferritic stainless steel connector substrate as the cathode and graphite as the anode, a multi-stage cathode micro-arc electrodeposition is performed in a manganese-cobalt-cerium composite electrolyte to form a Mn-Co-Ce precursor coating; the manganese-cobalt-cerium composite electrolyte contains 20 g / L manganese sulfate, 20 g / L cobalt sulfate, 5 g / L cerium sulfate, and 40 mg / L complexing agent (hydroxyethylidene diphosphonic acid), and the solvent is deionized water; the specific method of the multi-stage cathode micro-arc electrodeposition is as follows: 1) One-stage constant current cathode micro-arc electrodeposition: DC current density is 5A / dm 2 The time is 5 minutes; 2) Two-stage pulsed current cathode micro-arc electrodeposition: peak current density is 1.0 A / dm² 2 The positive and negative pulse frequencies are 300Hz, the duty cycle is 80%, and the duration is 60min. (3) The ferritic stainless steel connector with deposited Mn-Co-Ce precursor coating is placed in a protective atmosphere for homogenization heat treatment. The specific method of homogenization heat treatment is as follows: the temperature is uniformly raised to 700℃ at a heating rate of 1℃ / min and held for 10h, then the temperature is uniformly raised to 1000℃ at a heating rate of 20℃ / min and held for 10min; the temperature is lowered to 700℃, and then air is introduced and the oxidation treatment is carried out in the air atmosphere for 6h so that the Mn-Co-Ce precursor coating is converted in situ into Ce modified Mn-Co-O spinel composite coating. The Ce element is partially dissolved in the Mn-Co-O lattice, and the remaining Ce element is dispersed in the grain boundary region in the form of CeO2 nanoparticles to construct a composite barrier layer that inhibits the external diffusion of Cr and the internal diffusion of O. The surface morphology of the Ce-modified Mn-Co-O spinel composite coating in this embodiment is shown below. Figure 1 ,from Figure 1 As can be seen from the data, the surface of the composite coating exhibits spinel structure characteristics, is uniform and dense, and has no cracks or pores. The XRD pattern of the Ce-modified Mn-Co-O spinel composite coating in this embodiment is shown below. Figure 2 ,from Figure 2 As can be seen from the data, the composite coating is mainly composed of the (MnCoCe)3O4 phase.

[0019] Example 2: A method for preparing a Ce-modified Mn-Co-O spinel composite coating, the specific steps of which are as follows: (1) The ferritic stainless steel connector substrate is subjected to mechanical grinding, degreasing and cleaning, and pickling and activation (activated with 5wt% dilute sulfuric acid at 10℃ for 10min) to obtain a surface-pretreated ferritic stainless steel connector substrate. (2) Using a surface-pretreated ferritic stainless steel connector substrate as the cathode and graphite as the anode, a multi-stage cathode micro-arc electrodeposition is performed in a manganese-cobalt-cerium composite electrolyte to form a Mn-Co-Ce precursor coating; the manganese-cobalt-cerium composite electrolyte contains 50 g / L manganese sulfate, 5 g / L cobalt sulfate, 0.05 g / L cerium sulfate, a complexing agent (5 mg / L hydroxyethylidene diphosphonic acid and 60 mg / L sodium citrate), and deionized water as the solvent; the specific method of the multi-stage cathode micro-arc electrodeposition is as follows: 1) One-stage constant current cathode micro-arc electrodeposition: DC current density is 10 A / dm 2 The time is 1 minute; 2) Two-stage pulsed current cathode micro-arc electrodeposition: peak current density is 3.0 A / dm² 2 The positive and negative pulse frequencies are 1000Hz, the duty cycle is 30%, and the duration is 20min. (3) The ferritic stainless steel connector with deposited Mn-Co-Ce precursor coating is placed in a protective atmosphere for homogenization heat treatment. The specific method of homogenization heat treatment is as follows: the temperature is uniformly raised to 900℃ at a heating rate of 5℃ / min and held for 4h, then the temperature is uniformly raised to 1100℃ at a heating rate of 10℃ / min and held for 2min; the temperature is lowered to 900℃, and then air is introduced and the oxidation treatment is carried out in the air atmosphere for 2h so that the Mn-Co-Ce precursor coating is transformed in situ into Ce modified Mn-Co-O spinel composite coating. The Ce element is partially dissolved in the Mn-Co-O lattice, and the remaining Ce element is dispersed in the grain boundary region in the form of CeO2 nanoparticles to construct a composite barrier layer that inhibits the external diffusion of Cr and the internal diffusion of O. The cross-sectional morphology of the Ce-modified Mn-Co-O spinel composite coating in this embodiment is shown below. Figure 3 ,from Figure 3 As can be seen from the data, the composite material is uniform and dense, without cracks or pores, and the coating is tightly bonded to the stainless steel substrate, with Mn, Co, Ce and O evenly distributed. The Ce-modified Mn-Co-O spinel composite coating Ce-occurrence state analysis TEM image of this embodiment is shown below. Figure 4 ,from Figure 4 As can be seen from the diagram, the coating is dissolved in the Mn-Co-O lattice, causing it to be distorted, and Mn, Co, Ce and O are uniformly distributed.

[0020] Example 3: A method for preparing a Ce-modified Mn-Co-O spinel composite coating, the specific steps of which are as follows: (1) The ferritic stainless steel connector substrate was mechanically ground, degreased and cleaned, and then acid-washed and activated (activated in 3wt% dilute hydrochloric acid at 30℃ for 8 min) to obtain a surface-pretreated ferritic stainless steel connector substrate. (2) Using a surface-pretreated ferritic stainless steel connector substrate as the cathode and graphite as the anode, a multi-stage cathode micro-arc electrodeposition is performed in a manganese-cobalt-cerium composite electrolyte to form a Mn-Co-Ce precursor coating; the manganese-cobalt-cerium composite electrolyte contains 50 g / L manganese sulfate, 15 g / L cobalt sulfate, 4 g / L cerium sulfate, and complexing agents (30 mg / L hydroxyethylidene diphosphonic acid, 40 mg / L sodium citrate, and 55 mg / L trisodium phosphate), and the solvent is deionized water; the specific method of the multi-stage cathode micro-arc electrodeposition is as follows: 1) One-stage constant current cathode micro-arc electrodeposition: DC current density is 8A / dm 2 The time is 4 minutes; 2) Two-stage pulsed current cathode micro-arc electrodeposition: peak current density is 2.0 A / dm² 2 The positive and negative pulse frequencies are 800Hz, the duty cycle is 50%, and the duration is 45min. (3) The ferritic stainless steel connector with deposited Mn-Co-Ce precursor coating is placed in a protective atmosphere for homogenization heat treatment. The specific method of homogenization heat treatment is as follows: the temperature is uniformly raised to 850℃ at a heating rate of 4℃ / min and held for 6h, then the temperature is uniformly raised to 1050℃ at a heating rate of 15℃ / min and held for 5min; the temperature is lowered to 800℃, and then air is introduced and the oxidation treatment is carried out in the air atmosphere for 5h so that the Mn-Co-Ce precursor coating is converted in situ into Ce modified Mn-Co-O spinel composite coating. The Ce element is partially dissolved in the Mn-Co-O lattice, and the remaining Ce element is dispersed in the grain boundary region in the form of CeO2 nanoparticles to construct a composite barrier layer that inhibits the external diffusion of Cr and the internal diffusion of O. The Ce valence state analysis of the Ce-modified Mn-Co-O spinel composite coating in this embodiment is shown in [reference needed]. Figure 5 ,from Figure 5 It can be seen that Ce 2+ Ce 3+ and Ce 4+ The presence of Ce oxides is evident from the valence state of O; The weight gain during high-temperature oxidation of the Mn-Co coating used in the comparative example is compared with that of the Ce-modified Mn-Co-O spinel composite coating in this embodiment. Figure 6 High-temperature ASR comparison can be found in Figure 7 The oxidation weight gain of the Ce-modified Mn-Co-O spinel composite coating was reduced by about 3 times, and the ASR was reduced by about 1 time, remaining at 23.4 mΩ·cm. 2 The following significantly improves the high-temperature performance of SOFC stainless steel connectors.

[0021] The specific embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for preparing a Ce-modified Mn-Co-O spinel composite coating, characterized in that, The specific steps are as follows: (1) The ferritic stainless steel connector substrate is successively ground, degreased and cleaned, and pickled and activated to obtain a surface-pretreated ferritic stainless steel connector substrate; (2) Using a surface-pretreated ferritic stainless steel connector substrate as the cathode and graphite as the anode, a multi-stage cathode micro-arc electrodeposition was carried out in a manganese-cobalt-cerium composite electrolyte to form a Mn-Co-Ce precursor coating. (3) The ferritic stainless steel connector with deposited Mn-Co-Ce precursor coating is placed in a protective atmosphere for homogenization heat treatment, and then placed in an air atmosphere for oxidation treatment so that the Mn-Co-Ce precursor coating is transformed in situ into Ce modified Mn-Co-O spinel composite coating. The Ce element is partially dissolved in the Mn-Co-O lattice, and the remaining Ce element is dispersed in the grain boundary region in the form of CeO2 nanoparticles to construct a composite barrier layer that inhibits the external diffusion of Cr and the internal diffusion of O.

2. The method for preparing the Ce-modified Mn-Co-O spinel composite coating according to claim 1, characterized in that: Step (1) The acid is dilute hydrochloric acid or dilute sulfuric acid, and the concentration of the acid solution is 1~5wt%.

3. The method for preparing the Ce-modified Mn-Co-O spinel composite coating according to claim 1 or 2, characterized in that: Step (1) The pickling and activation temperature is 10~40℃ and the time is 1~10min.

4. The method for preparing the Ce-modified Mn-Co-O spinel composite coating according to claim 1, characterized in that: Step (2) The manganese-cobalt-cerium composite electrolyte contains 20-50 g / L manganese sulfate, 5-20 g / L cobalt sulfate, 0.05-5 g / L cerium sulfate, and 40-130 mg / L complexing agent. The solvent is deionized water. The complexing agent is one or more of hydroxyethylidene diphosphonic acid, sodium citrate, and trisodium phosphate.

5. The method for preparing the Ce-modified Mn-Co-O spinel composite coating according to claim 1, characterized in that: The specific method for multi-stage cathode micro-arc electrodeposition in step (2) is as follows: 1) One-stage constant current cathode micro-arc electrodeposition: DC current density of 5~10 A / dm 2 The time is 1 to 5 minutes; 2) Two-stage pulsed current cathode micro-arc electrodeposition: peak current density is 1.0~3.0 A / dm² 2 The positive and negative pulse frequencies are 300~1000Hz, the duty cycle is 30%~80%, and the duration is 20~60 min.

6. The method for preparing the Ce-modified Mn-Co-O spinel composite coating according to claim 1, characterized in that: The specific method for homogenization heat treatment in step (3) is as follows: heat the temperature at a rate of 1~5℃ / min to 700~900℃ and hold for 4~10h, then heat the temperature at a rate of 10~20℃ / min to 1000~1100℃ and hold for 2~10min.

7. The method for preparing the Ce-modified Mn-Co-O spinel composite coating according to claim 6, characterized in that: The specific method for oxidation treatment in step (3) is as follows: cool down to 700~900℃, then introduce air and keep warm for 2~6 hours.

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