Preparation And High Temperature Performances Of Co-Mn₃O₄ Composite Coating

Solid oxide fuel cell(SOFC)is a new type of power generation device that converts chemical energy into electrical energy.With its all-solid structure,low pollution and high efficiency,it has been widely concerned by human.Since a single battery can only provide a voltage of about 1 V,it is necessary to assemble the single battery into a battery stack with a interconnect to meet the power supply requirements in the use of large-scale electrical equipment.It can be seen that the interconnect plays a key role in SOFC.With the transition of SOFC from high temperature to low temperature,perovskite ceramic materials as the traditional interconnect are replaced metal materials.Ferritic stainless steels(FSS)have been considered as the most promising interconnect material for intermediate temperature SOFC.However,there are two problems in the process of high temperature oxidation,the first one is that the volatilization of Cr in the metal matrix will lead to "cathode Cr-poisoning" and the second one is that in the process of oxidation,the gradually thickened Cr2O3 scale will lead to the gradual increase of eletrical resistance,which are detrimental to the performance of SOFC.In order to solve the above problems,it is necessary to prepare a protective coating on the surface of the interconnect to hinder the vaporization of Cr and improve its electrical conductivity.In this paper,Co-Mn3O4 composite coating was prepared on the surface of stainless steel by electrophoresis-electroplating deposition method.Then subjected to thermal conversion to form a protective coating based spinel MnCo2O4.First,a layer of Co is pre-plated on the surface of SUS430 stainless steel.And then Mn3O4 particles are electrophoretically deposited on the surface of the Co-coated steel.Finally,followed by subsequent electroplating process of Co.Co-Mn3O4 composite coatings are prepared on the surface of stainless steel.In this process,the influence of electroplating process parameters(plating temperature,bath pH,cathod current density and electroplating time)on the composite coating Co-Mn3O4 are explored.Then,the coated stainless steels are oxidized in air at 800℃.The short-term oxidation behavior and long-term oxidation behavior of the oxidized coated stainless steels are evaluated and test the electrical properties of the surface oxide scale.The experimental results are as follows:(1)The optimal electroplating process parameters of obtaining Co-Mn3O4 composite coating are as follows:bath temperature 40℃,bath pH 9,cathode current density 15 mA·cm-2,plating time 10 min.And the mass fraction of Mn3O4 in the composite coating is 36.84%.Under this condition,the adhesion between the composite coating and substrate is good and the Mn3O4 particle content is the highest.(2)The results of short-term oxidation show that after thermal exposure for 5 h,the oxide scale surface presents a two-layer structure,including the inner layer of Cr2O3 and the outer layer of MnCo2O4/Co3O4.After thermal exposure for 100 h,the oxide scale is divided into three layers:Cr2O3 inner layer,(Co,Mn,Fe)3O4 middle layer,MnCo2O4 outer layer.The content of Co3O4 gradually decreases due to the participation of oxidation to form spinel coating.Meanwhile,spinel coating can well inhibit the vaporization of Cr scale.After thermal exposure in air at 800℃ for 100 h,the area specific resistance(ASR)of the oxide scale is 11.124 mΩ·cm2.Electrical conductivity of the oxide scale is good.(3)The results of long-term oxidation show that after oxidized for 1 week,3 weeks,and 5 weeks,the oxide scale showed a three-layer structure,namely the inner layer of Cr2O3,the middle layer of(Co,Mn,Fe)3O4,and the outer layer of MnCo2O4.Spinel coating can effectively inhibit volatilization of Cr.From the cross-section morphology,we can easily observe some holes in the oxide scale.The Cr2O3 layer gradually thickens with the increase of oxidation time,so the ASR of the oxide scale also gradually increases with the increase of oxidation time.But all are less than 100 mΩ·cm2,which meets the electrical requirements of SOFC interconnect.