Influence of Mn–Co Coating on Area Specific Resistance of SUS441 and Cathode Side Interfacial Resistance

Solid oxide fuel cell (SOFC) stacks depend on metallic interconnects to ensure the performance with cathode-side protection and interfacial resistance as key determinants. The existing coatings like perovskite, active element oxides and spinel all have drawbacks. For instance, perovskite coatings fail to inhibit oxygen ion diffusion, while active element oxides are typically thin and porous. In contrast, spinel coatings have a promise due to their lower densification temperature and ability to suppress chromium volatility. However, there is little work on the contact resistance. In this work, a Mn–Co spinel coating was applied to SUS441 stainless steel to reduce both oxidation and interfacial resistance. This approach could offer a viable solution for optimizing the performance of SOFC stacks.

SUS441 was chosen as an interconnect substrate material, and all the samples were cut to small pieces with the dimensions of 10.0 mm × 10.0 mm × 1.5 mm. Manganese nitrate and cobalt nitrate (AR, Sinopharm Group Co., Ltd., China) were dispersed in ethanol solvent, with dispersant, binder, and defoamer added to prepare a precursor solution. A wet-spray device was used to deposit Mn_1.5Co_1.5O₄ coating on the SUS441 substrate (coated SUS441), which was then dried in air and calcined at 850 ℃ for 10 h.

The area-specific resistance (ASR) of the samples was measured by a four-terminal method with a CHI760e electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., China), with a voltage range of –1 to 1 V. The interfacial contact resistance was calculated based on the ASR values.

The thermal expansion behavior was characterized by a model DIL402C dilatometer (Netzsch Co., Germany) at a heating rate of 5 ℃/min. The microstructures were determined by a model S4800 scanning electron microscope (Hitachi Co., Japan). The oxidation mass gain was monitored by a model FA2104 electronic balance (Shanghai Shunyu Hengping Scientific Instruments Co., Ltd., China). A three-dimensional surface profile was obtained by a model LSM700 laser confocal microscope (Zeiss Co., Germany).

The coated SUS441 exhibits an uneven surface but a dense cross-section free of cracks or bubbles, indicating a robust adhesion between the coating and substrate. The Mn-Co spinel coating substantially reduces the interconnect’s resistance of 0.47 mΩ·cm², compared to 3.29 mΩ·cm² for uncoated SUS441. Also, the coating enhances the activation energy (E_a) for electrical conductivity from 0.59 eV to 0.91 eV, thereby slowing electron migration and reducing the oxidation rate.

The results of long-term testing at 750 ℃ show that the ASR of uncoated SUS441 increases from 26.07 mΩ·cm² to 52.41 mΩ·cm² after 720 h, then gradually decreases and stabilizes at 25.54 mΩ·cm². This may result from the maturation and reconstruction of the oxide layer, which lowers the resistance. In contrast, the coated SUS441 has an initial ASR of 3.96 mΩ·cm², which increases to 16.38 mΩ·cm² after 300 h and then slowly declines to 11.97 mΩ·cm² at 820 h. These findings demonstrate that the Mn–Co coating significantly enhances the high-temperature oxidation resistance of SUS441.

The oxidation mass gain is a key indicator of interconnect oxidation resistance. The coated and uncoated SUS441 interconnects both exhibit a rapid mass gain in the first 200 h. After 200 h, the mass gain rate of the coated SUS441 slows and stabilizes, whereas the uncoated SUS441 further increases. The dense Mn–Co coating effectively blocks oxygen penetration and metal ion migration, thereby slowing oxidation and enhancing resistance. The relationship between the square of oxidation mass gain and time indicates a change in oxidation behavior after approximately 200 h, likely due to the thickening of the oxide layer hindering further oxidation. As the oxide layer thickens, the internal mass transfer becomes more difficult, slowing the oxidation rate. The K_p value decreases by one order of magnitude during the slow oxidation phase, compared to the initial rapid stage.

The coating has a limited impact on the thermal expansion of the interconnect but brings the expansion coefficient of SUS441 closer to that of the La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) cathode, thus improving the thermal compatibility. The initial interfacial contact resistance between SUS441 and LSCF is 3.54 mΩ·cm², stabilizing at 14.57 mΩ·cm². For coated SUS441 and LSCF, the initial interfacial contact resistance decreases from 20.78 mΩ·cm² to 7.99 mΩ·cm². The Mn–Co coating likely improves the interface microstructure, reducing defects and roughness, and enhancing electron transfer. The analysis of laser confocal microscopy indicates that the roughness matching value between SUS441 and LSCF is 0.605, and it is 1.156 for coated SUS441, indicating better surface compatibility and contact with LSCF.

The Mn–Co spinel coating was coated on the surface of SUS441 interconnect, and its effect on suppressing the interconnect’s oxidation behavior and reducing interfacial resistance were investigated. The results showed that the coating significantly reduced the ASR of SUS441 from 3.29 mΩ·cm² to 0.47 mΩ·cm² at 800 ℃, maintaining a relatively low ASR of 11.97 mΩ·cm² after 800 h oxidation. The results of oxidation mass gain tests indicated that the coating effectively reduced the oxidation rate of coated SUS441, which was only 43% of that of the uncoated sample. For the interfacial properties, the contact resistance between the Mn–Co coated SUS441 and the LSCF cathode was 7.99 mΩ·cm², which was just 55% of that of the uncoated sample, with improved interfacial roughness matching. These findings could demonstrate that the Mn–Co spinel coating effectively mitigated the oxidation rate and interfacial resistance of SUS441, thereby enhancing its high-temperature oxidation resistance.