Invited Review: Some recent developments in the atomic-scale characterization of structural and transport properties of ceria-based catalysts and ionic conductors

Abstract

Ceria-based materials are utilized as automotive exhaust catalysts for the removal of noxious compounds, as catalysts for reforming ethanol and methane to produce hydrogen in fuel cells, as materials for solar-energy-to-fuel conversion, and as cathode, anode, and electrolyte materials in solid oxide fuel cells (SOFCs). The present paper is a critical review on the atomic-scale characterization of oxide-ion diffusion pathway, the existing phases, the phase transformations, “metastable” and stable phase diagrams, and oxygen storage capacity (OSC) of ceria-based materials. “Metastable,” compositionally (x) homogeneous t′- and t″-ceria–zirconia CexZr1–xO2 solid solutions (0.2<x<0.9) are key materials to obtain a high OSC, leading to high catalytic activity. Here, the t′- and t″-forms are unstable compared to the two-phase mixture of stable ZrO2-rich tetragonal and CeO2-rich cubic (or t″) phases, but are stable in the partitionless, compositionally homogeneous phases. The axial ratio, c/aF where the subscript F represents the pseudo-fluorite lattice, of the t′-form is larger than unity, while the c/aF ratio of the t″-form equals unity. Formation of the t′- and t″-CexZr1–xO2 is depicted in the “metastable” phase diagram consisting of allotropic phase boundaries in the CeO2–ZrO2 system and is explained using the schematic Gibbs energy–composition (G–x) diagram. The composition (CeO2 content x)-induced t′–t″ transition in CexZr1–xO2 is discrete and of first order. The c–t″ phase transition of CexZr1–xO2 is induced by the oxygen displacement from the regular fluorite position 1/4,1/4,1/4 along the c-axis. The c–t″ transition is continuous and might be of higher order. The c–t″ phase boundary at room temperature is located at around x=0.85–0.9 in both bulk and nanocrystalline CexZr1–xO2. The tetragonal symmetry of compositionally homogeneous nano-sized Ce0.5Zr0.5O2 in air is retained up to 1176K. The c/aF ratio, and the oxygen displacement are smaller in the nanocrystalline Ce0.5Zr0.5O2 than in bulk Ce0.5Zr0.5O2. Bulk oxide-ion diffusion is an important step of oxygen storage and release in ceria-based catalysts, and the oxide-ion diffusivity is essential for high OSC and efficiency of the SOFCs and the solar-energy-to-fuel conversion. The present paper reviews the atomic-scale characterization of ion diffusion in ceria-based catalysts and fluorite-type ionic conductors and discusses the correlation between their bulk ion diffusivity and structural properties. The spatial distributions of neutron scattering length density, bond valence sum (BVS), and bond-valence-based energy (BVE) in the unit cell of tetragonal ceria–zirconia compounds, cubic fluorite-type ceria-based materials, and other fluorite-structured compounds such as Ce0.5Zr0.5O2, CeO2, ceria–yttria Ce0.97Y0.07O1.96, bismuth oxide solid solution δ-Bi1.4Yb0.6O3, and copper iodide α-CuI indicate the three-dimensional network of curved 〈100〉F ion diffusion pathways and anisotropic 〈111〉F thermal vibration of mobile ions, which are responsible for the bulk ion diffusion and conduction. Here, the subscript F denotes the pseudo-fluorite lattice. The BVE distributions of Ce0.5Zr0.5O2 and CeO2 indicate lower activation energy and higher mobility of oxide ions in Ce0.5Zr0.5O2 compared with CeO2.