Preparation And Property Optimization Of Co-Mn Spinel Coating On Surface Of Ferrite Alloy

The single cell of Solid Oxide Fuel Cell（SOFC）produces low voltages and therefore requires interconnect to build up the battery stack to meet practical requirements.With the development of SOFC technology towards medium and low temperatures,it has become possible to use metallic materials as interconnect instead of expensive ceramic materials.High performance and low-cost interconnect are key to the commercialization of SOFC.Ferritic Stainless Steel（FSS）with thermal expansion coefficient（CTE）that matches the battery components,high electrical conductivity,thermal conductivity and excellent mechanical performance,has become the most potential application of the interconnect material.However,under SOFC operating conditions,FSS tends to exhibit poor oxidation resistance and the outward volatilization of Cr leading to cathode poisoning,making SOFC performance gradually decay,greatly limiting the commercial development of SOFC.For these problems of FSS metallic interconnect,this thesis uses a composite electrodeposition technique that is apt to commercial production to prepare a composite coating of Co-Mn oxides on the surface of the SOFC commercial Crofer 22 APU interconnect,and then explores a thermal conversion method to obtain a（Co,Mn）₃O₄ spinel coating.On this basis,the coating is modified by doping conductive metal elements Cu and active element oxide Y₂O₃.Scanning electron microscopy（SEM）,Transmission electron microscopy（TEM）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and Area-specific resistance（ASR）were used to characterize and analyze the prepared Co-Mn and doped Co-Mn spinel coatings.The results show that（Co,Mn）₃O₄ spinel coating can effectively protect the Crofer 22 APU alloy.At the same time,the（Co,Mn）₃O₄ spinel coating with double doping of Cu+Y has the best performance in all aspects,which greatly improves the oxidation resistance,Cr resistance and high temperature conductivity of the coating,providing a feasible and effective way to prepare composite spinel coating on SOFC metallic interconnect.The main research contents and results are as follows:（1）The pretreatment method of ferrite alloy surface was optimized.The oxide film on the surface of Crofer alloy was removed by electrolysis corrosion,and then preplating was performed to prepare for the subsequent composite electrodeposition,which could effectively improve the bonding strength between the composite deposited coating and the matrix.（2）The orthogonal test was designed to obtain the optimal process parameters of composite electrodeposition（Mn oxide content 120 g/L,electrolyte p H=6,stirring speed 500 rpm,electrolyte temperature 40℃and cathodic current density of 20 m A/cm²）.According to the optimal process parameters,the Co-Mn oxide composite electrodeposition coating with good adhesion to the substrate was prepared by composite electrodeposition method.（3）A stepwise thermal conversion method was determined（heating to 600℃at a heating rate of 5℃/min,holding for 6 hours,then heating to 800℃at the same heating rate,and cooling in the furnace after holding for 4 hours）,which can effectively improve the performance of（Co,Mn）₃O₄ spinel coating.After500 hours of isothermal oxidation,the atomic proportion of Cr element diffused to the substrate surface is kept at 3%-4%,the oxidation weight gain is 0.74mg·cm^-2,and the ASR is 38.2 mΩ·cm².It inhibits the outward volatilization of Cr,optimizes the high-temperature oxidation resistance and conductivity,thus effectively prolonging the service life of Crofer alloy.（4）A simple and effective method for preparing Cu+Y double doped（Co,Mn,Cu,Y）₃O₄ spinel coating（CMCu Y）on the ferritic alloy Crofer 22 APU is provided.The ASR of the sample with CMCu Y coating after isothermal oxidation at 800℃for 500 hours is 5.89 mΩ·cm²,and after oxidation at 40000hours,the ASR of the sample is 52.68 mΩ·cm² calculated by the formula,which meets the service requirements of SOFC.At the same time,after 500 hours of isothermal oxidation,the atomic proportion of Cr element diffused to the substrate surface reaches 2.17%,and the proportion of Cr⁶⁺/Cr^total on the coating surface is stable at about 25%,which can effectively inhibit the volatilization of Cr.The oxidation weight gain is 0.167 mg·cm^-2 at 500 hours,and the oxidation rate is 4.0×10^-16 g² cm^-4 s^-1,which has a good antioxidant effect.（5）The model of Cu+Y double doped spinel crystal was established and optimized,and the conduction mechanism of Cu+Y double doped spinel crystal on atomic scale was revealed through first principles calculation.The results show that Cu+Y doping reduces the band gap of the system from 0.57 e V to0.11 e V.It is found that Cu provides empty orbit near the bottom of the conduction band,and Cu and Y together provide free electrons at the top of the valence band,indicating that the doping of Cu+Y can reduce the activation energy of electronic transition and improve the conductivity of the spinel system.