Coated Stainless Steel 441 as Interconnect Material for Solid Oxide Fuel Cells: Evolution of Electrical Properties

摘要

In recent years there has been extensive research about metallic interconnects for SOFCs. Besides chromium based alloys, ferritic stainless steels (FSS) have been commonly suggested for meeting the materials requirements for SOFC applications. Examples of FSS alloys specially developed for this use are Crofer 22 H, ZMG 232 and Sanergy HT. Metallic interconnects face three major material challenges; degradation due to oxidation, evaporation of chromium which poisons the cathode and the degradation of the electrical conductivity because of the growing oxide scale. For a sufficient performance regarding these three issues, protective coatings have to be applied - this leads to additional costs. For cost savings cheaper base materials such as AISI 441 coated with protective coatings seem to be promising. We investigated AISI 441 coated with a double layer coating of 10 nm cerium (inner layer) and 630 nm cobalt. The material was exposed to a simulated cathode atmosphere of air with 3 % water at 850 °C and the samples were exposed for up to 1500 h. The performance of chromium evaporation, corrosion rate and electrical degradation was monitored. The electrical properties could be linked to the evolving microstructure during the different stages of exposure. Both the degradation of the electric performance and the oxygen uptake (mass gain) followed a similar trend. Chromium evaporation could be decreased by 90 % compared to the uncoated substrate material.