CHEN Bangfu ^{1, 2}, CHEN Yunxia ¹, YU Yutian ², WU Bo ^{2, 3}, LIN Youchen ²,
LIU Qing ⁴, GUAN Chengzhi ², WANG Jianqiang ²
(1. School of Materials Science and Engineering, Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China; 2. Department of Hydrogen Energy Technology, Shanghai Institute of Applied Physics, Chinese Academy of Science, Shanghai 201800, China; 3. College of Science, Shenyang University of Chemical Technology, Shenyang 110027, Liaoning, China; 4. Shanghai EGEN Energy Technology Co., Ltd., Shanghai 201800, China)

Extended Abstract: [Background and purpose] Solid oxide fuel cell (SOFC) is a highly efficient electrochemical device that can be used to directly convert chemical energy of hydrogen and hydrocarbon fuels into electricity in an environmentally friendly manner. While the typical operating temperatures of SOFCs have been reduced to 600–850 ℃, ferritic stainless steels (FSS), such as ZMG 232, AISI 441, SUS430, etc., can be utilized as interconnect materials, due to their high electrical conductivity, low material/manufacturing cost, excellent mechanic strength and similar coefficient of thermal expansion (CTE) to other cell components. However, it still exists several issues during their operation at 600–850 ℃, including continuous growth of Cr₂O₃ scale, Cr migration to cathode and dimensional tolerance at interconnect-cathode interface. These issues will cause serious performance degradation for stacks. To solve these problems, a dense protective coating and a porous contact layer are typically prepared between the metallic interconnect and the cathode. The dense protective coating is utilized to inhibit the growth of Cr₂O₃ scale and prevent Cr migration to the cathode, while the porous contact layer can provide better electrical pathway to decrease the power losses and compensate for the dimensional tolerance between the interconnect and the cathode.[Methods] In this study, MnCoNiFeCu alloy powder was selected as the precursor materials for dense protective coating and porous contact layer. A coating/contact dual-layer structure was fabricated through reactive co-sintering in the simulated SUS 430 interconnect/coating/contact/cathode/cathode support test cells. The precursors were calcined in air at 900 ℃ for 2 h to study phase evolution and microstructure of the protective coating and the contact layer. The phases in the sintered layers were characterized by using X-ray diffraction (XRD), whereas scanning electron microscopy (SEM) featured with energy-dispersive spectroscopy (EDS) was utilized to analyze the microstructure and obtain the compositional information. The test cells were fabricated to examine the electrical performance of the dual-layer structure via Area-Specific Resistance (ASR). After the initial sintering, the furnace temperature was then dropped to 800 ℃ for isothermal oxidation for 1000 h and ten thermal cyclic test. The tested samples were then epoxy-mounted, sectioned, and polished for examining cross-sectional microstructure after oxidation. EDS line scans near at the interconnect-coating and contact-LSM cathode interfaces were obtained to identify the possible interdiffusion between the cell components. Furthermore, the effectiveness of the dual layer in inhibiting the growth of the Cr₂O₃ layer and blocking Cr migration from the interconnect to cathode was also assessed.[Results] XRD results of the protective coating and contact layer after thermal conversion revealed that the sintered samples predominantly consisted of spinel phase, with minor oxides, while no metallic phases were detected. Specifically, the protective coating exhibited a majority of the spinel phase along with CuO and CoO, as confirmed by EDS mappings, indicating the aggregation of Cu and Co on the surface. In contrast, the contact layer primarily contained the spinel phase and CuO, with EDS mappings indicating uniform distribution of Fe, Co, Ni, Mn and Cu, while no delamination was observed. In ASR measurements, the tested sample exhibited stable behavior with an ASR of only 22.04–22.71 mΩ·cm² during the 1000 h isothermal oxidation, while the thermal cycling led a dramatic increase in ASR. Once the ASR measurement was completed, the sample cross-sectional surface was characterized. For isothermal oxidation, a dense protective coating and a relatively porous contact layer were observed between the interconnect and cathode. The dual-layer structure was well-boned with the interconnect and cathode after the isothermal oxidation, indicating that the double-layer structure exhibited exceptional thermal compatibility with the adjacent cell components. Importantly, no Cr was detected within both the contact layer and cathode, further confirming the effectiveness of the double-layer structure in inhibiting the migration of Cr from the interconnect to cathode. Conversely, the thermal cycling test sample exhibited serious cracking at the interface between the porous contact layer and the LSM cathode, which was responsible for the rapid increase in ASR during thermal cycling testing.[Conclusions] In this study, MnCoNiFeCu alloy powder was utilized as the precursor material to develop a dense protective coating and a porous contact layer simultaneously through reactive co-sintering, forming a (Mn,Co,Ni,Fe,Co)₃O₄-based dual-layer structure. The sample exhibited stable behavior with an ASR of only 22.04–22.71 mΩ·cm² after 1000 h of isothermal oxidation, while the thermal cycling led a dramatic increase ASR in. Notably, the growth of the Cr₂O₃ scale was dramatically suppressed, while no Cr was detected in the LSM cathode, confirming the effectiveness of the thermally converted dual-layer structure in blocking the migration of Cr. The HEA used as the dense protective coating and porous contact layer offers several advantages, including uniform microstructure, improved electrical performance, exceptional Cr-blocking capability and simple fabrication process. In the future study, more efforts should focus on optimizing the elements in the precursor alloy to further enhance the uniformity and CTE matching of the dual-layer structure after sintering.
Key words: solid oxide fuel cell; high-entropy alloy; reactive co-sintering; spinel; dual-layer structure