Study on Exsolution Process of Sr2FeMo0.6Ni0.4O6 via in Situ Cathodic Polarization

Abstract

Solid oxide cells (SOCs) are nowadays one of the most promising energy conversion technologies, to accelerate and promote the ongoing energy transition 1,2 , based on the use of renewable resources. These devices in fact allow the development of valuable low carbon footprint power-to-X (X= power, fuels) chains of energy conversion and storage 3 . In this respect is crucial the design of innovative, cost-effective materials and processes for more and more versatile and reversible devices. In the last decade, simple perovskite (ABO 3 ) and double perovskite (A 2 BBʹO 6 ) oxide have been proved to be a valuable alternative to cermet SOC electrodes, thanks to their relative ease of functionalization via doping and exsolution and their mixed ionic-electron conduction 4-6 . Exsolution process is strongly dependent on the type of metal and perovskite and on methodology adopted to induce the reduction 7 . Cathodic electrochemical polarization has been demonstrated to be a valuable approach to boost the exsolution especially from titanate based structures, obtaining higher dispersions than that derived from thermal reduction 8 . In this work, we explored for the first time the behaviour of the double perovskite Sr 2 FeMo 0.6 Ni 0.4 O 6-δ (SFMN) under cathodic polarization and we investigated the impact of the structural evolution on the electrochemical performances of a multi-functional electrode for H 2 -SOFC and CO 2 -SOEC applications. SFMN was prepared by sol gel method and used to prepare supported SFMN/ La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3-δ (LSGM)/La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ :Ce 0.9 Gd 0.1 O 2-δ (LSFCo:GDC) cells that were tested either before and after thermal or electrochemical reduction at 850°C. As already reported the thermal reduction of SFMN leads to the exsolution of metal nanoparticles (of Ni or Ni-Fe alloys) and the in situ formation of Ruddlesden-Popper phase 9 (RP). HRTEM, SEM and XRD characterizations of tested cells allowed to observe an acceleration of the structural transformation of perovskite under cathodic polarization in comparison to what observed under thermal reduction. This allows to gain insights on the role of entire transformation on the electrochemical behaviours of cells. Electrochemical properties of SFNM were investigated by EIS analysis. Distribution of relaxation times (DRT) analyses was also used to obtain further insights on the impedance of the different cell mechanisms according to their characteristic frequency. The exsolved metal nanoparticles contributed to improve the conductivity and activity of the electrode, however, also the formation of RP phase seems have a significant role, especially in the electroreduction of CO 2 . Further studies are in progress to better understand the mechanisms of interaction between the phases formed during the exsolution process and their role on SOC electrodes activity. References 1 Hauch et al., Science 370, eaba6118 (2020). 2 M.B. Mogensen et al. Clean Energy, 3 (2019) 175–201 3 F. Salomone et.al Chem. Eng. Journal 377 (2019) , 120233 4.Irvine, J. T. S. Perovskite Oxide Anodes for SOFCs. In Perovskite Oxide for Solid Oxide Fuel Cells ; Ishihara, T., Ed.; Springer US: Boston, MA, 2009; pp 167–182. 5. W. Yin, et al. Energy Environ. Sci. 12 ( 2019 ) 442–462 6. Q. Islam, et al. J. Power Sources 492 ( 2021 ) , 229626. 7. O. Kwon et.al. J. Phys. Energy2, ( 2020) , 032001 8. Jh, Myung, Jh. et al. Nature 537, (2016) 528–531 9. Z.; Du et.al ACS Nano 10 , 2016 , 8660–8669