Effect of Electrodeposited Nickel Coatings on the High Temperature Degradation and Electrical Performance of Steel SOEC Interconnects

Abstract

The performance of solid oxide electrolyzer cells (SOEC) can be improved through the development of coatings applied to the surface of ferritic steel interconnects in view of mitigating chromium evaporation and reducing the growth rate of low conductive oxides in oxidizing environments. This work investigated the oxidation and area specific resistance (ASR) of two electrodeposited nickel coatings on preoxidized and non-preoxidized AISI 441 ferritic stainless steel substrates. The nickel coating effectively restricted the outward diffusion of chromium after 100 h of exposure at 700 °C in air but led to nickel/iron interdiffusion between the substrate and coating forming an iron-nickel-rich spinel on the surface, with NiO underneath and Cr₂O₃ at the coating-substrate interface and at the coating grain boundaries. The application of a LSM ((La_0.80Sr_0.20)_0.95MnO_3−x) coating on top of the Ni electrodeposited coatings resulted in the same type of oxides but the oxidation kinetics were slower. Interdiffusion continued with the exposure at 700 °C for 2400 h resulting in the growth of a thick iron-rich oxide layer on top of Cr₂O₃, steadily raising the interconnect ASR to 25 mΩ cm². The addition of a preoxidation step before the electrodeposit of nickel helped to limit iron-nickel interdiffusion, leading to the formation of a thicker NiO layer on a Cr₂O₃ layer between substrate and coating. While the ASR was higher than without preoxidation at the beginning of the test, it stabilized at about 33 mΩ cm² after 1750 h. Despite displaying a higher electrical resistance, the coatings effectively limited the outward chromium diffusion throughout exposure compared to the bare substrate.