Investigation On Preparation And Properties Of NiCo₂O₄-based Spinel Coating For SOFC Metallic Interconnect

Nowadays,effective measures such as reduction of electrolyte thickness,using of new electrode,electrolyte materials and preparation techniques are advocated by many researchers aiming to reduce the high operating temperature of solid oxide fuel cell（SOFC）from temperatures above 1000℃to 600-800℃.Higher electronic conductivity,lower cost and easier fabrication make ferritic stainless steel to replace traditional ceramic materials as SOFC interconnects.However,there are some defects with ferritic stainless steel as interconnects at the SOFC operating temperature.Such as poor high-temperature oxidation resistance,cathode poisoning caused by volatile Cr,high area specific resistance,which seriously affect the electrical properties of the interconnect and consequently degrade the performance of the battery stack.In this paper,Ni-Co,Ni-Co-Fe and Ni-Co-Cu alloy coatings of suitable composition and thickness were deposited on the surface of SUS430 ferritic stainless steel by electrodeposition process,respectively.The thermal transformation was simulated in SOFC working environment to study the oxidation resistance and electrical properties of Ni-Co alloy and Ni-Co alloy modified with appropriate amount of Fe and Cu as protective coatings for SOFC interconnects,respectively,aiming to obtain modified interconnects with excellent overall performance.In the high-temperature air atmosphere,the Ni-Co alloy was oxidized to form Ni Co₂O₄spinel,and part of the outward diffusion of Co was oxidized to form Co₃O₄spinel.Meanwhile,part of the inward diffusion of Ni reacted with the outward diffusion of Fe in the substrate to form Ni Fe₂O₄spinel.The spinel layer in the oxide coating could maintain excellent stability,which not only prevented the outward diffusion of Cr,but also isolated the Cr-rich oxide layer from the high-temperature air,thus effectively improving the oxidation resistance and inhibiting the volatilization of Cr to the SOFC cathode.However,holes and pores tended to form along the substrate/oxide coating interface due to elemental inter-diffusion.The addition of Fe or Cu to the Ni-Co alloy coating inhibited the outward diffusion of Cr and reduced the area specific resistance（ASR）.The addition of Fe and Cu changed the growth mechanism of the oxide layer on the surface of Ni-Co coating sample,reduced the interfacial pores due to elemental inter-diffusion by blocking the transformation of Ni Fe₂O₄spinel,and improved the oxide layer adhesion.The reduction of interfacial pores and the formation of stable spinel oxide layers effectively limited the inward diffusion of O and the outward diffusion of Cr.The oxidation rate of Ni-Co-Cu coated samples was less than that of Ni-Co coated samples and Ni-Co-Fe coated samples during the oxidation time of 0-1000 h.The oxidation mass gain rate of Ni-Co-Cu coated sample was 7.47×10^-14g²·cm^-4·s^-1within the oxidation time of 100-1000 h,which was 28%and 20%lower compared to the Ni-Co coated sample and Ni-Co-Fe coated sample,respectively.It indicated that the addition of Cu to the Ni-Co alloy as a protective coating for the SOFC metal interconnects could further improve the oxidation resistance of the interconnect.The performance optimization was better compared with the addition of Fe to Ni-Co alloy.The ASR of the Ni-Co-Cu coated sample increased from 10.12 mΩ·cm²to 13.44mΩ·cm²after oxidation from 20 h to 1000 h in air at 800℃.The theoretical value of ASR of the Ni-Co-Cu coated sample was only 57.45 mΩ·cm²after 40,000 h of oxidation,which was 35%and 15%lower than that of Ni-Co coated samples and Ni Co-Fe coated samples,respectively.It indicated that the addition of Cu to the Ni-Co alloy could greatly improve the electrical properties of the interconnect and its enhancement was superior to that of the Ni-Co and Ni-Co-Fe alloy coatings.