Advanced characterization and functional evaluation of Ni-/Fe-doped Cu–Mn spinels as Co-free coatings for SOFC/SOEC interconnects

Abstract

To develop Co-free, high-performance coatings for SOEC/SOFC interconnects, Ni- and Fe-doped Cu_1.3Mn_1.7O₄ spinels were synthesized via an EDTA-assisted gel method and applied onto low-chromium ferritic steel via electrophoretic deposition. Ni²⁺ preferentially substitutes Cu²⁺ in octahedral positions, increasing lattice parameters and improving electrical conductivity and corrosion resistance, while doping with Fe introduces structural disorder and secondary phases. DFT calculations confirm Ni and Fe's preference for octahedral Cu sites, aligning with experimental XRD, IR, and XAS analyses. Ni-doped spinels, particularly CuMn_1.7Ni_0.3O₄, exhibit enhanced conductivity and thermal stability, and are therefore promising materials for SOEC/SOFC applications. Reactivity studies show reduced Cr₂O₃ interaction, with CuO segregation observed for 0.1 mol dopants. Long-term oxidation tests demonstrate excellent corrosion resistance and stable ASR values in the 14–20 mΩ cm² range after 3000 hours at 750 °C, with Ni-doped coatings performing best. Confocal Raman imaging further revealed structural variations and phase distribution within the coating layers, and TEM studies demonstrated the complexity of the coating/steel interface, including the formation of a nanocrystalline Cr₂O₃ interlayer and progressive chromium diffusion into the spinel lattice. These features suggest a chemically graded, yet structurally coherent interface contributing to long-term coating stability. These findings highlight Ni doping as an effective strategy for optimizing CM spinel coatings for high-temperature electrochemical applications.