Effect of grain boundaries on ion migration in stabilized $\delta-Bi_2O_3$ thin film electrolyte

Abstract

Solid electrolytes with high oxygen-ion conductivity are of significant interest for many applications such as solid-oxide fuel cells (SOFCs), gas sensors and permeation membranes. Over the past several decades, numerous studies have been conducted on the effect of grain boundaries on the process of increasing the ionic conductivity of solid electrolytes. Given that nanocrystalline thin- or thick-films have been investigated in relation to lowering the operating temperature of solid electrolytes to less than $500^\circ C$, more rigorous and quantitative assessments are necessary to determine how the ion transport characteristics are affected by the numerous interfaces formed from a high density of nano-grains. In this context, I selected highly conductive stabilized $\delta-Bi_2O_3$ as a target material and investigated the effect of grain boundary on ionic transport properties. More specifically, I focused on the oxygen ion conductivity and the phase stability that are closely associated with migration of anions and cations, respectively. The nano-polycrystalline thin films of yttria-stabilized $Bi_2O_3 (YSB)$ and erbia-stabilized $Bi_2O_3 (ESB)$ were prepared by the pulsed laser deposition (PLD), and the oxygen ion conductivity was analyzed by AC impedance spectroscopy (ACIS) as a function of temperature and oxygen partial pressure. Surprisingly, both epitaxial and polycrystalline YSB films show nearly identical levels of oxygen ion conductivity at elevated temperature ($350~500^\circ C$) despite the fact that the poly-film possesses an extremely high density of the grain boundaries. Furthermore, the epitaxial ESB thin film maintained cubic $\delta -phase$ for a long time (> 100h) at $600^\circ C$ without any conductivity deterioration, whereas the poly film exhibited abrupt phase transition and reduced conductivity similar to bulk ESB. These observations provide precise, quantitative understanding of grain boundary effects through well-defined control experiments and a new direction for utilizing a stabilized $Bi_2O_3$ as high-performance electrolytes for high-temperature electrochemical applications.