Interface Design Development for High Durability of SOFC Interconnectors Using Electrodeposition Coating

Abstract

ENE-FARM type S equipped with solid oxide fuel cell (SOFC) stack has been commercialized since 2012. We have been working on improving the power generation efficiency, reducing costs, and the making them more compact by considering that market expansion and cost reduction are critical for the continued market spread of ENE-FARM type S. However, there is a trade-off between cost reduction and performance improvement and issues related to achieving both have emerged during the development process. In order to reduce costs, significant reduction of the number of cells were required. By reducing the number of cells, the current per cell increased, and the stainless steel connecting the cells as the SOFC interconnector was required to have further improvement in durability at high temperature. The degradation modes of the stainless steel used for SOFC mainly include the growth of an insulating oxide layer such as Cr2O3, Cr poisoning of the SOFC cathode, and anomalous oxidation due to decrease in the Cr concentration in the stainless steel. Therefore, ceramic coating is indispensable for the SOFC interconnector in order to suppress these degradations. Material properties of the coating film and coating manufacturing conditions are critical to the ceramic coating equipped on the SOFC interconnector. In addition, it has excellent physical properties such as high Cr diffusion suppression, barrier performance of oxygen gas diffusion, and good physical properties such as high electrical conductivity and good matching of thermal expansion coefficient with the SOFC components. In general, spinel-based oxide materials have been reported as the coating materials for SOFC interconnector, and many binary oxides containing transition metals such as MnCo2O4, MnCu2O4, and NiCo2O4. We have investigated (Zn, Mn, Co)3O4 because of matching the thermal expansion coefficient to the ferritic stainless steel and improving the sinterability at lower temperature. The total electrical conductivity and the thermal expansion coefficient were changed due to the Mn / Co ratio. In terms of these properties, the optimum composition in ZnMnCoO4 was found to be the Mn / Co ratio of around 1 / 1. We have evaluated the long-term durability of SOFC stack components for 12 years in a short period of time by temperature acceleration evaluation method. The voltage changes with the constant current density of 0.3 A cm-2 at a furnace temperature of 973 K shows no degradation over 45000 hours. Moreover, ZnMnCoO4 coated on the 22Cr-Fe alloy shows higher durability over 20000 hours at 1123 K. In 2016, we released the ENE-FARM type S equipped with the electrodeposition coating technology and achieved the highest power generation efficiency and the smallest size at that time. The electrodeposition coating technology has been used in all of ENE-FARM type S sold in Japan. Furthermore, this electrodeposition technology has contributed significantly to the increase in sales volume since 2016. The electrodeposition coating technology can be applied not only to SOFC but also SOEC and is expected to be used more and more widely in the future.