Highly conductive and stable Mn1.35Co1.35Cu0.2Y0.1O4 spinel protective coating on commercial ferritic stainless steels for intermediate-temperature solid oxide fuel cell interconnect applications

Abstract

Chromia scale growth and Cr evaporation of ferritic stainless steel interconnects are known to be major causes of serious degradation of the solid oxide fuel cell (SOFC) stack. The development of suitable ceramic coating materials on the metallic interconnects has been demonstrated as an effective way to address these challenges. Herein, we developed a Mn1.35Co1.35Cu0.2Y0.1O4 (MCCY) spinel material via a facile glycine-nitrate process as a protective coating on a metallic interconnect (SUS 441). Crystal structure and surface charge state analysis of the MCCY material revealed that co-doping of Y and Cu into the (Mn,Co)3O4 spinel resulted in redistribution of the Mn ions (Mn3+ and Mn4+) into the octahedral site, which increased the electrical conduction by enhanced small polaron hopping. Accordingly, the MCCuY-coated interconnect exhibited ∼8 times lower area specific resistance (ASR) than that of the undoped Mn1.5Co1.5O4 (MCO) coated interconnect. Moreover, time-dependent ASR behavior of MCCuY-coated sample was monitored in-situ using electrochemical impedance spectroscopy at 650 °C, showing excellent stability with no observable change for >1000 h, while the ASR of the MCO-coated sample was raised by ∼71%. After 1000 h operation, we found strong adhesion between the MCCuY coating and the metallic interconnect as well as remarkably restricted Cr diffusion into the coating layer. Furthermore, the parabolic constant associated with the oxidation kinetics of the MCCuY-coated substrate (8.25 × 10−11 mg2 cm−4 s−1) was ∼1 order of magnitude lower than that of the MCO-coated one (7.34× 10−10 mg2 cm−4 s−1) at 650 °C after 1000 h measurement. These results demonstrate that the MCCuY is a highly promising coating material of metallic interconnects for intermediate-temperature SOFC applications.