Abstract
Detrimental to the performance of solid oxide cells (SOCs) is the interface between electrolyte and each electrode: for purely electron-conducting electrodes, it is part of the triple phase boundary where the reactions take place, and for mixed ionic and electronic conductor (MIEC) electrodes, the oxygen ions coming from or moving into the electrolyte are transported through this phase boundary. As such, the performance of SOFCs and SOECs is strongly dependent on the chemical nature of this interface.
By combining state-of-the-art electron microscopy, synchrotron-based X-ray spectroscopy and theoretical calculations, we were able to obtain an atomically resolved picture of the interface between yttria-stabilized zirconia (YSZ, with 8 mol% Y2O3) and lanthanum strontium manganite (LSM, (La0.8Sr0.2)0.95MnO3-δ).[1]
Energy-dispersive X-ray (EDX) spectroscopy measurements in the transmission electron microscope (TEM) reveal the presence of a significant amount of inter-diffusion between YSZ and LSM. Only strontium does not show any diffusion into YSZ, resulting in a shift of its intensity distribution profile of approximately 0.8 nm with respect to all other elements. All this is in agreement with Monte-Carlo-based simulations that were used to theoretically model the YSZ/LSM boundary including the ion- as well as site-specific swapping probabilities for the diffusion process.[1] According to these simulations, this diffusion region is slightly amorphous. By means of atomically resolved scanning TEM (STEM), this reduction of long-range order was observed experimentally for an approximately 1.5 nm wide slab on the YSZ side of the boundary, clearly distinguishing it from the bulk fluorite structure.
In materials science, distinct '2D-like' layers at grain boundaries and interfaces are often referred to as 'complexions'. While these can in general occur in different types, depending on thickness and retained order,[2] they are all characterized by a self-limited (finite) width and thermodynamic stability obtained only by confinement in between two bulk phases (i.e. they cannot exist on their own without neighboring phases).[2] These complexions have recently also been discovered in other energy-related systems such as battery materials.[3]
The presence of such a stable complexion might be the reason why the YSZ/LSM interface is not as prone to the formation of lanthanum or strontium zirconates as other perovskites such as lanthanum strontium cobalt ferrite (LSCF).[4] Consequently, it might have significant influence on the chemical stability of such SOCs and a thorough understanding of the complexions between electrolyte and electrode may in the future allow fine tuning of SOC performance and stability. This requires the determination of electrochemical and thermodynamic properties of these interface complexions for which we will, in a next step, employ theoretical simulations and experimental techniques in order to find out how different transport phenomena, widths and stabilities of the layers behave with temperature, time and chemical environment.
[1] H. Tuerk, F.-P. Schmidt, T. Götsch et al. in preparation
[2] S. J. Dillon et al. Acta Mater. 2007, 55, 6208–6218
[3] J. Timmermann et al. Phys. Rev. Lett. 2020, 125, 206101
[4] W. Wang et al. J. Electrochem. Soc. 2006, 153, A2066