The increase in environmental awareness has led to the significant worldwide trend of the development of alternative energy sources other than conventional use of fossil fuels. One of the alternatives is the use of Solid Oxide Fuel Cells (SOFCs). However, usage of fuels other than hydrogen (e.g. biogas) is burdened with carbon deposition and sulfur poisoning when conventional Ni-YSZ cermet is used as an anode, leading to rapid decrease in cell efficiency. Among materials studied as substitutes for Ni-YSZ, perovskites are one of the most popular group. Sr₂Fe_1.5Mo_0.5O₆ (SFM) is one of the representatives of the double perovskites family extensively examined due to its high ionic and electronic conductivity, as well as the redox stability in both oxidizing and reductive atmospheres. However, it is possible that under highly reductive atmospheres, ferrites may transform into other phases, causing severe distortion of the compound [1], thus a deeper understanding of the phase transition should be considered in the case of SFM-base compunds.
Herein, the La dopant is introduced on the A-site of SFM as well as nickel and cobalt are added on the B-site, resulting in formation of the La_0.3Sr_1.7Fe_1.425Mo_0.475M_0.1O₆ (LSFM, where M=Ni or Co). Lanthanum was chosen as a dopant to increase the electronic conductivity of the compound [2], while transition metals are meant to undergo the exsolution phenomenon and enhance catalytic properties. The reference sample without additional transition metal was also prepared. Upon the reduction at 800 °C in hydrogen the samples underwent the phase transition from double perovskite to the Ruddlesden-Popper layered perovskite. The SEM imaging has revealed that the phase transition is followed by the formation of well-dispersed nanoparticles covering the grains’ surface. The reversibility of the aforementioned transition was studied by reoxidizing samples at equal temperature. XRD proved that after such treatment, the double perovskite was partially or even completely (LSFM_Co) restored. At the same time, previously formed nanoparticles were no longer observed during SEM imaging. The temperature of the aforementioned transition was determined by the DSC and High Temperature XRD studies. The impact of introducing different dopants into the structure on electrical properties was examined by the DC4W method in both air and hydrogen. Due to the high possibility of both Fe and Mo to be in different oxidation states, X-ray Absorption Spectroscopy measurements were performed on the samples in all three states: as-prepared, reduced and reoxidized to reveal the oxidation state of B-site elements. All presented studies provide a deep insight view into the phase transition phenomena in La-doped SFM double perovskite.
Acknowledgments:
The research project was supported by the National Science Center under grant No. NCN 2022/45/N/ST5/02933. We acknowledge SOLARIS Centre for the access to the PIRX beamline.
Literature:
[1] H. Lv, L. Lin, X. Zhang, D. Gao, Y. Song, Y. Zhou, Q. Liu, G. Wang, X. Bao, J. Mater. Chem. A 7 (2019) 11967–11975.
[2] X. Yang, J. Chen, D. Panthi, B. Niu, L. Lei, Z. Yuan, Y. Du, Y. Li, F. Chen, T. He, J. Mater. Chem. A 7 (2019) 733–743.