| Solid oxide fuel cell(SOFC)is a solid-state energy conversion device which can convert chemical energy stored in fuel directly into electrical energy by electrochemical reaction.The interconnect is one of the key components of SOFC,which plays a dual role in connecting the single cell and separating cathode and anode gases.The metallic interconnect replaces the traditional ceramic interconnect due to its low cost,easy fabrication,high electrical and thermal conductivities.Ferritic stainless steel has become preferred metal interconnect material of SOFC stack due to the low cost,compatible thermal expansion coefficient(CTE)with the other parts of SOFC stack,good oxidation resistance as well as good electrical conductivity.However,the problems in hindering its application include the fast growth of Cr2O3 sub-layer,the increase of resistance and cathode Cr-poisoning,which can lead to a serious decline in the performance of SOFC stack.At present,an effective way to solve these problems is to apply a coating that can act as a protective layer on the metallic interconnect surface.The coatings are developed in order to reduce the oxidation rate of metallic interconnect,reduce the contact resistance and prevent the outward diffusion of Cr,thereby maintaining long-term stability of the SOFC performance.In this thesis,Cu coating,Cu-Mn3O4 composite coating and Cu-Mn3O4-CeO2 composite coating were developed on the SUS 430 ferritic stainless steel by means of electrodeposition.The high-temperature oxidation behaviors of coated steels were investigated and the electrical performance of the oxide scale on the coated steels was tested.20 g·L-1 Mn3O4 particles were added to copper sulfate solution with pH of 3 and bath temperature of 35℃,and Cu-Mn3O4 composite coating was prepared on SUS 430 ferritic stainless steel by means of composite electrodeposition at the cathode current density of 2.12 A·dm-2.A dual-layer oxide scale formed on the steel with Cu-Mn3O4 composite coating after oxidation for 50 h at 800℃ in air consisted of an external layer of CuO,(Cu,Mn)3O4 spinel,(Cu,Fe)3O4 spinel and an internal layer of Cr2O3,indicating that the oxide scale formed on the coated steel inhibited the outward diffusion of Cr.The suitable parameters for Cu-Mn3O4 composite electrodeposition in alkaline copper pyrophosphate solution,such as C6H12N4 concentration in the bath(10 g·L-1),bath temperature(50℃),current density(12.5 mA·cm-2),Mn3O4 concentration in the bath(20 g·L-1)and pH(9 or 9.5)were determined.Under the optimum conditions,the mass fraction of Mn3O4 in the composite coating obtained was 8.02%,the absolute value of zeta potential of Mn3O4 particle,cathodic overpotential and Faraday resistance reached the maximum and the critical load(Lc)of composite coating obtained was 21.4 N.The morphology,composition and phase structure of the oxide scale formed on the steels with Cu coating and Cu-Mn3O4 composite coating obtained under the optimum conditions were characterized by SEM,EDS and XRD,and the electrical performance of the surface oxide scale formed on the coated samples were tested.The experimental results showed that the steel with Cu coating experienced an initially large mass gain and then the mass gain increased slightly after the first-week rapid oxidation stage,and the oxidation rate constant(0.052 mg2·cm-4·w-1)of the coated steel was lower than that(0.073 mg2·cm-4.w-1)of the uncoated steel;after thermal exposure,a dual-layer oxide scale consisting of an external layer of CuO and an internal layer.of Cr2O3 was primarily formed on the steel with Cu coating,and a spinel layer consisting of Cu,Cr,Fe and Mn was formed at the CuO/Cr2O3 interface;the scale ASR value of the steel with Cu coating after 5-week oxidation was 9.72 mΩ·cm2 at 800℃,which was significantly lower than that of the uncoated steel;after the first-week rapid oxidation,the mass gain of the steel with Cu-Mn3O4 composite coating increased at a relatively low rate,and its oxidation rate was similar to that of the uncoated steel;a dual-layer oxide scale mainly comprising an external layer of CuO followed by(Cu,Mn,Fe)3O4 spinel and an internal layer of Cr-rich oxide was thermally developed on the steel with Cu-Mn3O4 composite coating;the steel with Cu-Mn3O4 composite coating exhibited lower scale ASRs compared with the uncoated steel for the same test conditions,and the scale ASR values of the coated steels after 1 and 5-week oxidation were found to be as low as 17.93 mΩ·cm2 and 22.46 mΩ·cm2,respectively,at 800℃.In order to further improve the oxidation resistance of the coated steel and the electrical conductivity of the oxide scale formed on the coated steel,CeO2 powder which contains active element was added to the alkaline Cu-Mn3O4 electroplating bath and the Cu-Mn3O4-CeO2 composite coating was deposited on SUS 430 ferritic stainless steel.The high-temperature oxidation behaviors and electrical performance for the CeO2-doped coated steel were evaluated.The experimental results showed that the steel with Cu-Mn3O4-CeO2 composite coating initially experienced a rapid mass gain,and then mass gain increased slightly after the first-week rapid oxidation stage;a dual-layer oxide scale formed on the steel with Cu-Mn3O4-CeO2 composite coating after 1 and 5-week oxidation was composed of an external layer of CuO,(Cu,Mn,Fe)3O4 spinel,CeO2 and an internal layer of Cr2O3;the Cu-Mn3O4-CeO2 coated steel exhibited the scale ASR value as low as 20.09 mΩ·cm2 at 800℃,which was lower than that of the Cu-Mn3O4 coated steel(22.46 mΩ·cm2)and the uncoated steel(35.86 mΩ·cm2);the oxide scale thermally converted from the Cu-Mn3O4-CeO2 composite coating reduced the growth rate of Cr-rich oxide internal layer,effectively inhibited the outward diffusion of Cr and improved the electrical performance. |
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