Research Progress of Modified Mn-Co Spinel Coating of SOFCs Metal Interconnect

Fe-Cr ferrite stainless steel is an ideal material for the interconnects of low and medium temperature solid oxide fuel cells (SOFCs), but its surface is easily oxidized in the high temperature aerobic environment and can cause "Cr poisoning" of the cathode, resulting in the reduction of cell efficiency. Mn-Co spinel is widely used as protective coatings for the interconnects to improve the oxidation resistance of the interconnects and reduce the diffusion of Cr. However, in the long-term service process of the Mn-Co spinel coating, there is still a phenomenon that the diffusion of Cr element leads to the continuous thickening of the Cr2O3 transition oxide layer and the decrease of the electrical conductivity of the coating. It is found that the above problems can be effectively improved by doping modification of Mn-Co spinel coatings. In this study, based on the research progress of Mn-Co spinel coating in recent years, the typical Mn-Co spinel crystal structure and its conduction mechanism were briefly described, and the possible doping sites of modified elements in Mn-Co spinel and their effects on the crystal structure of spinel were summarized. The effects of the modification of rare earth elements Y, La, Ce and transition group elements Cu and Fe on the oxidation resistance, electrical conductivity, adhesion and thermal expansion coefficient compatibility of Mn-Co spinel coatings were emphatically expounded, and the mechanism of modification elements was summarized. The focus of different elements on the modification of Mn-Co spinel coatings was summarized and compared. Finally, the problems existing in the Mn-Co spinel coating in the current research and the research direction of the modified Mn-Co spinel coating in the future were prospected. The doping of rare earth elements in the Mn-Co spinel coating can improve the denseness of the coating and reduce the thickness of the Cr2O3 transition oxide layer, thereby improving the electrical conductivity and oxidation resistance of the interconnects. In addition, the doping of rare earth elements in the coating can inhibit the formation of defects at the oxide/substrate interface and improve the bond strength between the coating and the substrate. Therefore, rare earth elements have a dramatic improvement on the oxidation resistance, electrical conductivity and adhesion of coating. Fe doping into Mn-Co spinel can make its coefficient of thermal expansion more compatible with the ferrite stainless steel substrate, but reduce the conductivity of the coating. Doping Cu in the coating can significantly enhance its electrical conductivity, and the Cu elements can stabilize the spinel crystal structure and prevent the structural transformation from causing large thermal stress caused by the structural transformation which leads to the cracking of the coating. In the current study, the results of the mechanism of Y, La and Ce in Mn-Co spinel coatings are roughly the same, but the mechanism of rare earth elements in Mn-Co spinel coatings lacks a clear understanding compared to Cu and Fe doped Mn-Co spinel coatings. Therefore, the construction of the theoretical model of rare earth doping is still the focus of research to be solved in the coming period of time. The mechanism of action of Cu and Fe doping in Mn-Co spinel coatings and the effect of coating properties become clearer, but Cu and Fe doping can only solve the electrical conductivity and coefficient of thermal expansion of the coatings, respectively. Therefore, the development of new modification means or new coating preparation processes to simultaneously improve the electrical conductivity and coefficient of thermal expansion matching of Mn-Co spinel coatings is also one of the difficulties to be overcome in the research. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.