Optimization of ionic conductivity in doped ceria

Abstract

Oxides with the cubic fluorite structure, e.g., ceria (CeO2), are known to be good solid electrolytes when they are doped with cations of lower valence than the host cations. The high ionic conductivity of doped ceria makes it an attractive electrolyte for solid oxide fuel cells, whose prospects as an environmentally friendly power source are very promising. In these electrolytes, the current is carried by oxygen ions that are transported by oxygen vacancies, present to compensate for the lower charge of the dopant cations. Ionic conductivity in ceria is closely related to oxygen-vacancy formation and migration properties. A clear physical picture of the connection between the choice of a dopant and the improvement of ionic conductivity in ceria is still lacking. Here we present a quantum-mechanical first-principles study of the influence of different trivalent impurities on these properties. Our results reveal a remarkable correspondence between vacancy properties at the atomic level and the macroscopic ionic conductivity. The key parameters comprise migration barriers for bulk diffusion and vacancy-dopant interactions, represented by association (binding) energies of vacancy-dopant clusters. The interactions can be divided into repulsive elastic and attractive electronic parts. In the optimal electrolyte, these parts should balance. This finding offers a simple and clear way to narrow the search for superior dopants and combinations of dopants. The ideal dopant should have an effective atomic number between 61 (Pm) and 62 (Sm), and we elaborate that combinations of Nd͞Sm and Pr͞Gd show enhanced ionic conductivity, as compared with that for each element separately. density functional theory ͉ diffusion ͉ point defects ͉ solid oxide fuel cells ͉ CeO 2 M aterials providing high conductivity of oxygen ions are urged by a number of important technological applications, such as oxygen sensors and solid oxide fuel cells (1). The latter are expected to become high-efficiency electrical power generators that enable clean energy production and support sustainable development (2-4). A standard electrolyte for solid oxide fuel cell applications is yttria-stabilized zirconia (YSZ) (1, 5). To increase the ionic conductivity of YSZ to a technologically useful level, high operating temperatures (Ϸ1,000°C) are required. Lowering of the operating temperatures would considerably increase the applicability and competitiveness of solid oxide fuel cells. The ionic conductivity ( ) can be expressed as an exponential function of the activation energy for oxygen vacancy diffusion (E a ),where T stands for temperature, k B for the Boltzman constant, and 0 for a temperature-independent prefactor. Materials with lower E a will facilitate ionic conductivity at lower temperatures, and here rare-earth doped ceria is one of the main candidates (1, 4). The basic principle for the choice of a dopant, advocated by many researchers, is the ability of the dopant to minimize the internal strain of the lattice (6-8). Clear understanding of the phys...

Impact of doping on the ionic conductivity of ceria: A comprehensive model

“…[18][19][20][21][22] Andersson and co-workers have investigated the O migration and the vacancy-dopant association for low dopant concentrations. 23 Similar concentrations have been studied by Nakayama and co-workers. 24 The aim of the present work is to understand the impact of dopants on the electronic structure and ionic conductivity for technologically relevant dopant concentrations.…”

The nature of oxygen states on the surfaces of CeO2 and La-doped CeO2

“…[6] It is generally considered that when CeO 2 is doped with a trivalent cation, such as La(III), two lattice Ce(IV) ions are replaced by the dopants and an O ion is removed to conserve the charge. [22,23] In Kröger-Vink notation, this is:…”

Electronic Structures and Oxygen Ion Migration Energies of Metal Doped CeO2Systems：A DFT+U Study

“…考虑到 Ce 和 Sm 含有高角量 子数 4f 电子而 DFT 方法对这些电子间相互作用描述不 足的原因, 本文采用引入 Hubbard 参数 U 的方法, 其中 Ce 的 U 值选取 6.0 eV, 其他理论研究表明, Ce 的 U 值 应大于 5 eV [20] [20,27] . [12,31,32] . …”