Effects of Transition Metals on the Electrical Conductivity of M-Doped MnCo₂O₄ (M = Cu, Ni, Zn) as Contact Layer on Precoated SUS441 in Solid Oxide Cells

Spinels have been widely employed as contact layers (CLs) on metallic interconnects for solid oxide cells (SOCs) with the required electrical conductivity and Cr block ability. In this work, MnCo2O4 spinels doped with different transition metals (Cu, Ni, and Zn) were deposited on precoated SUS441 stainless steel as CLs. A positive doping effect of Cu and Ni on increasing the concentration of Mn4+/Mn3+ and Co3+/Co2+ hopping pairs was observed, whereas a negative doping effect of Zn on electrical conductivity was confirmed, which was reflected in the data of X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectra (XAS). The Cu-doped Mn1Co1.7Cu0.3O4 spinel exhibited the best electrical conductivity with a low area specific resistance (ASR) value of 4.5 mΩ·cm2 at 850 °C. A relative degradation test demonstrated that Mn1Co1.7Cu0.3O4 maintained significant performance during a 200 h isothermal process. Cross-sectional observations revealed that the thickness of the Cr oxide scale formed during the degradation test was much reduced by the Mn1Co1.7Cu0.3O4 CL, which was attributed to the improved resistance to Cr oxidation. The present work shed light on the design of highly conductive CLs in SOCs.