Dopant Induced-Modulation in Reducing Ability of Cerium in Doped Ceria System and Its Effect on Oxy-Ion Conductivity: Core Study by XPS and XANES Probes

Abstract

Acceptor-doped ceria systems are potential materials forelectrolytes in intermediate temperature solid oxide fuel cells (IT-SOFCs), but the reduction of Ce4+ to Ce3+ in the ceria-based electrolytes limits such functionality. Reduction is accompanied by the release of one electron + oxygen vacancy; leading to electronic contribution and making short circuiting critical. Suppressing the reducing ability of cerium is thus a core challenge in optimizing its use. We developed acceptor (Sm3+, Gd3+) singly and doubly doped ceria, as valence and size of dopants are key in altering ionic conductivity. The oxidation state of dopant and host is revealed by XPS spectroscopy and XANES. XPS confirms the existence of Sm in two states, while Gd exists in one. The existence of Ce in Ce3+ and Ce4+ states are confirmed by both techniques, which also show that Ce4+ content in Sm-Gd doubly doped system is more than singly doped, and pure ceria, i.e. reduction ability of cerium in ceria is found to be suppressed by Sm and Gd double dopant. Effects of doping on the geometric and electronic structure have been studied using first-principles density functional theory. Ionic conductivity of a doubly doped system is enhanced by one order with suppression of reduction of cerium. The study shows acceptor multiple dopants in ceria system have potential to suppress reducing ability of cerium and improve ionic conductivity at intermediate temperature (400 °C–700 °C).