Protective role of Cu coatings on SUS441 interconnects in the anode atmosphere of direct methanol solid oxide fuel cells

The oxidation behaviors of bare and Cu-coated SUS441 ferritic stainless steel were systematically investigated in methanol-steam atmospheres at 650 °C and 750 °C, conditions relevant to the anode of direct methanol solid oxide fuel cells (SOFCs). The bare and Cu-coated (5, 10, and 15 μm) SUS441 alloy were evaluated in terms of oxidation kinetics, microstructural evolution, and electrical performance over 1000 h. The results showed that bare SUS441 exhibited typical parabolic oxidation, forming a multi-layered oxide scale dominated by Cr 2 O 3 , MnCr 2 O 4 , and Fe 3 O 4 , leading to a substantial increase in area-specific resistance (ASR) beyond the practical threshold. In contrast, Cu-coated specimens demonstrated significantly reduced oxidation rates and superior electrical stability. Among the tested coatings, the 10 μm layer exhibited the most favorable balance between protection and material economy, maintaining an ASR below 4.0 mΩ cm 2 after 1000 h at 650 °C. The improved performance is attributed to the selective barrier effect of Cu against oxidizing species and its ability to hinder Cr outward diffusion, although agglomeration at 750 °C partially compromised surface coverage. Cross-sectional analysis revealed the formation of a composite oxide (Cr 2 O 3 –Fe 3 O 4 ) beneath the Cu layer, indicating that oxidation was mitigated but not entirely suppressed. These findings demonstrate that Cu electroplating is an effective and scalable approach for extending interconnect durability in methanol-fueled SOFCs, particularly at moderate operating temperatures. • High-purity Cu coating was applied on SUS441 alloy. • Structural integrity of Cu coating was maintained after 1000 h oxidation at 650 °C. • Cu coating thickness strongly impacted oxidation kinetics and electrical stability. • Cu-coating (10 μm) SUS441 exhibited ASR <4 mΩ cm 2 after 1000 h in methanol-water. • Predicted lifetime of Cu-coated SUS441 for direct methanol SOFCs reached 36,000 h.