In Situ TEM Study of Oxygen Surface Exchange on Ceria-Based Oxides

Oxygen exchange reactions play an important role in many energy conversion applications. For example, in solid oxide fuel cells (SOFC), at the cathode interface, molecular oxygen is first reduced to oxygen ions and then diffuses through a solid electrolyte to interact on the anode side for fuel oxidation [1]. Oxygen exchange reactions are complex and consist of multiple steps including O₂ adsorption/desorption, molecular dissociation/association, electron transfer, and incorporation/removal oxygen into or from surface oxygen vacancies. Nonstoichiometric oxides, such as CeO₂ and ceria-based doped materials, are ideal candidates for oxygen exchange applications, because of the ability to reversibly exchange lattice oxygen with the ambient environment while maintaining a stable structure [2]. Doping ceria with aliovalent cations, such as Gd and Pr, can significantly increase the oxygen vacancy concentration resulting in higher ionic conductivity [3]. Due to the complexity of the multi-step oxygen exchange reaction, the surface exchange properties and mechanisms with different dopants are not well understood. To develop a deeper understanding of the materials, it is necessary to study the surface oxygen vacancy creation and annihilation process of the materials under different oxygen partial pressure and temperature in environmental transmission electron microscope (ETEM).<br/><br/>Cube-shaped 15% Gd-doped ceria (GDC), 15% Pr-doped ceria (PDC) and pure ceria (CeO₂) were synthesized with a hydrothermal method [3]. Time resolved <i>in</i><i> </i><i>situ</i> aberration-corrected TEM was used to observe atomic level variations in the oxygen vacancy creation/annihilation activities on the material surfaces. High temporal resolution images were acquired on an FEI Titan ETEM 80-300 microscope using a newly installed Gatan K3 direct electron detector. Our preliminary results show the local oxygen vacancy activity can be directly interpolated from oxygen column intensity changes and also can be associated with fluxional behavior of the adjacent cations. Relaxation of up to 60pm in cation position was observed to occur when oxygen exchanged between the (110) surface lattice sites and the ambient environment at room temperature. Due to the complex behavior of intensity in the phase contrast image (e.g. varying by specimen tilt and thickness), a convolutional neural network was designed to make direct determinations of both oxygen and cerium column occupancies. Further in-situ TEM study will continue on GDC and PDC to exam the dopant effect on surface oxygen exchange mechanisms.<br/><br/>References:<br/>[1] Adler, S. B., Chemical Reviews, 104(2004), 4791–4844.<br/>[2] Trovarelli, A., “Catalysis by ceria and related materials”(Imperial College Press, London), 25<br/>[3] Mai, H., Journal of Physical Chemistry B, 109(2005), 24380-24385<br/>[4] The authors acknowledge funding from NSF (DMR-1840841, CBET-1134464, OAC-1940263), and the use of facilities of Eyring Materials Center, High Performance Computing Resources at Arizona State University