Cerium-based oxide (ceria) is among the most investigated materials in heterogeneous chemical catalysis as an active support and electrochemical catalysis as a non-metallic oxygen-ion-conducting electrode for applications such as three-way catalysts (TWCs) and solid oxide fuel cells (SOFCs). In such devices, ceria is typically polycrystalline and is used in close contact with other components at high temperatures (> 600°C). Thus, during device operation, cation impurities from other components can be diffused into the ceria, especially through grain boundaries, which are a fast cation transport path.
In this study, we investigated metal impurities (Ni, Pt, and Al) diffusion phenomenon through the grain boundaries of acceptor-doped ceria as a function of temperature, pO2, and the type and concentration of dopant using dense polycrystalline thin films prepared by pulsed laser deposition (PLD). The remarkably high grain boundary density of thin films with vertically-oriented, nanosized-columnar grains enables accurate analysis of the diffusion kinetics and the solubility of metal impurities by means of time-of-flight secondary ion mass spectroscopy (ToF-SIMS). It is revealed that impurities diffuse unexpectedly quickly, and a considerable amount of impurities dissolves inside ceria grain boundaries. Furthermore, we monitored how the oxygen-ion transport properties and surface oxidation reactivity of ceria change according to the presence of impurities (Co, Ni, Cu, Au, and Pt), demonstrating the importance of diffusion control of the impurities in the grain boundaries.