Electrophoretic co-deposition of Fe-doped MnCo spinel as protective coatings for Solid Oxide Fuel Cells interconnects.

Solid oxide fuel cells (SOFC) are considered a promising technology for the production of clean energy. Ferritic stainless steels are widely used as interconnects in the SOFC stack. The development of volatile Cr-containing species, which can poison the cathode, as well as the steel oxidation at high temperature, are the main causes of degradation of the device. Manganese cobalt spinels have been proposed as protective coating materials for metallic interconnects due to their high electrical conductivity and the good CTE match with the steel. The doping of the spinel structure by transition metals leads to the modification of its properties, with the possibility to improve the coating performances. In this work, undoped and Fe-doped MCO spinel coatings are produced, by electrophoretic co-deposition of Mn1,5Co1,5O4 and Fe2O3 as iron precursor; two suspensions with increasing amount of iron oxide are prepared. Crofer 22 APU, an alloy specially developed as SOFC interconnect, and the commercial AISI 441 are used as substrates for the deposition. The coatings are sintered by a two-step heat treatment, performed in different atmospheres and at different temperatures. Dense and continuous coatings are obtained. The achievement of the “in-situ” formation of the modified spinel is verified by XRD. The effect of the Fe-doping is studied for both the substrates in comparison with the undoped coating. The oxidation kinetics are evaluated by thermogravimetric analysis, up to 2000 h at 750 °C, demonstrating that the Fe addition remarkably reduces the parabolic oxidation rate for Crofer 22 APU. The electrical properties are tested in terms of area specific resistance (ASR), up to 3200 h at 750 °C, using La0,85Sr0,1Mn1,1O3 (LSM) contact plates, that simulate the interactions with the SOFC cathode. The results are compared with those obtained by ASR measurement with platinum contacts on the samples aged for 1000 and 2000 h at the same temperature. All Crofer 22 APU exhibits an increasing ASR starting from ca. 1500 h, likely due to the formation of a Cr-rich reaction layer between the oxide scale and the coating. Despite the microscopic analysis has revealed the presence of a continuous silica layer on the surface of the AISI 441 samples after 3200 h aging, this steel shows a better ASR stability over time at constant temperature. Based on the information obtained, two different oxidation mechanisms for Crofer 22 APU and AISI 441 are proposed.