Reactive magnetron sputtered (Mn, Co)₃O₄ spinel coatings for superior element diffusion suppression and conductivity in ferritic stainless-steel interconnects of solid oxide fuel cells

Depositing a protective coating on the interconnect is critical to ensure the long-term stability of solid oxide fuel cell (SOFC) stacks. In this study, (Mn, Co)₃O₄ spinel coating is prepared by reactive magnetron sputtering to suppress element diffusion from ferritic stainless steel (FSS) substrate. The phase composition and valence distribution of sputtered films under different flow ratios of Ar/O₂ are investigated. The amorphous films deposited by magnetron sputtering are annealed in air at 800 °C and cubic phase MnCo₂O₄ spinel coating is obtained. The dense film prepared by reactive magnetron sputtering effectively impedes the inward diffusion of O₂ during the annealing process and suppresses the formation of a low-conductivity Cr₂O₃ layer. In addition, Higher ratios of Mn³⁺/Mn⁴⁺ and Co²⁺/Co³⁺ cation pairs are observed in the MnCo₂O₄ film deposited with an Ar/O₂ flow ratio of 30/1, which is conducive to electron conduction. The oxidation resistance and electrical conductivity of the FSS substrate are improved with a dense MnCo₂O₄ coating in a thickness of 2.5 μm. The oxidation rates of the coated samples decrease significantly by 1.4 and 2.2 times after 1000 h oxidation at 650 °C and 750 °C, respectively. Meanwhile, the area specific resistance (ASR) reduces by 20 % at 650 °C and 34.1 % at 750 °C. Compared with the metallic mode, the reactive mode can avoid effectively the diffusion of Co to the substrate and Fe to the spinel coating during the long-term oxidation process, showing a high compositional stability.