Development of Promising Co-Free Spinel Oxides via Electrodeposition Coating for Solid Oxide Cell Stacks

Electrodeposited Co-free spinel ZnFeMnO 4 coating on ferritic stainless steel (SUS) was evaluated as an interconnect/separator material for solid oxide cell (SOC) and benchmarked against Co 2 MnO 4 . Variable-temperature X-ray diffraction (RT–800 °C) confirmed a phase-pure spinel and a tetragonal to cubic transition, with no impurity phases observed. Although ZnFeMnO 4 pellets exhibited an order-of-magnitude lower total conductivity than Co 2 MnO 4 between 600 and 900 °C, coated assemblies showed area-specific resistance (ASR) above 750 °C comparable to those of Co 2 MnO 4 , with higher ASR only below 700 °C. Isothermal oxidation in air at 900 °C followed stable parabolic kinetics without anomalous oxidation to 1500 h; mass gains of ZnFeMnO 4 - or Co 2 MnO 4 -coated SUS were less than 40% of that of uncoated SUS. Energy-dispersive X-ray mapping revealed no detectable Cr in the La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3− δ (LSCF) layer after 100–1200 h at 900 °C, attributable to a dense interfacial ZnFeMnO 4 sublayer that blocks outward chromium diffusion and inward oxygen ingress. At 900 °C for 2500 h, the ASR remained essentially constant with negligible increase. Under these conditions, the coating layer densified and the porous overlayer diminished, while Cr diffusion was suppressed and no delamination occurred. These results identify electrodeposited ZnFeMnO 4 as a durable Co-free coating that satisfies oxidation resistance, Cr-leakage suppression, and ASR stability requirements in SOC stacks.