Lanthanum oxide-coated stainless steel for bipolar plates in solid oxide fuel cells (SOFCs)

Solid oxide fuel cells typically operate at temperatures of about 1000 °C. At these temperatures only ceramic interconnects such as LaCrO3 can be employed. The development of intermediate-temperature solid oxide fuel cells (IT-SOFCs) can potentially bring about reduced manufacturing costs as it makes possible the use of an inexpensive ferritic stainless steel (STS) interconnector. However, the STS suffers from Cr2O3 scale formation and a peeling-off phenomenon at the IT-SOFC operating temperature in an oxidizing atmosphere. Application of an oxidation protective coating is an effective means of providing oxidation resistance. In this study, we coated an oxidation protective layer on ferritic stainless steel using a precursor solution prepared from lanthanum nitrate, ethylene glycol, and nitric acid. Heating the precursor solution at 80 °C yielded a spinable solution for coating. A gel film was coated on a STS substrate by a dip coating technique. At the early stage of the heat-treatment, lanthanum-containing oxides such as La2O3 and La2CrO6 formed, and as the heat-treatment temperature was increased, an oxidation protective perovskite-type LaCrO3 layer was produced by the reaction between the lanthanum-containing oxide and the Cr2O3 scale on the SUS substrate. As the concentration of La-containing precursor solution was increased, the amount of La2O3 and La2CrO6 phases was gradually increased. The coating layer, which was prepared from a precursor solution of 0.8 M, was composed of LaCrO3 and small amounts of (Mn,Cr)O4 spinel. A relatively dense coating layer without pin-holes was obtained by heating the gel coating layer at 1073 K for 2 h. Microstructures and oxidation behavior of the La2O3-coated STS444 were investigated.