Processing and performance of protective Ni-doped CuMn spinel interconnect coatings

Abstract

Stainless-steel porous substrates for metal-supported solid oxide fuel cells require protective coatings to prevent chromium poisoning of the cathode. In this study, CuNi_0.2Mn_1.8O₄ powders were synthesized by the glycine nitrate combustion synthesis process and protective coatings were deposited on porous SUS 430 substrates by electrophoretic deposition and densified using a two-step annealing procedure. It was found that an AC signal of 500 Hz, $\pm$20 V voltage amplitude with a 60/40 duty ratio (coating deposition to removal time ratio), combined with a stirring rate of 200 RPM, resulted in a ∼ 2 μm relatively uniform coating throughout the surfaces of the porous structure. The conductivity of CuNi_0.2Mn_1.8O₄ decreased and the activation energy of small polaron hopping increased with increasing Cr doping concentration. The diffusivity of Cr in CuNi_0.2Mn_1.8O₄ at 700°C was determined to be 7.93 × 10⁻²⁰ m²/s. It is predicted that the surface of a 2 μm CuNi_0.2Mn_1.8O₄ coating will not exceed the solubility limit of Cr even after 50,000 h of operation, highlighting the excellent gettering property of the coating. A model was developed that indicated that at 700 °C, the coating layer contribution to the area specific resistance is more dominant during the first 250 h, after which the contribution of the Cr₂O₃ layer becomes more significant. Compared to the uncoated sample, the ASR of the coated metal support is expected to be less than 1/10th of that of an uncoated sample after 50,000 h of operation. These results show that AC-EPD CuNi_0.2Mn_1.8O₄ coatings not only mitigates chromium poisoning in SOFC stacks but also maintains robust electrical conductivity, thereby promising enhanced long-term cell performance.