Defect and Transport Model of Ceria–Zirconia Solid Solutions: Ce_0.8Zr_0.2O_2−δ—An Electrical Conductivity Study

Abstract

The electrical conductivity (σ) of Ce 0.8 Zr 0.2 O 2−δ was measured and modeled using a defect chemical-based model including contributions from singly and doubly ionized oxygen vacancies, acceptor impurities, and small polaron electrons. By analyzing the pO 2 dependence of σ in terms of the defect model, a transition between an impurity dominated regime at high pO 2 and lower temperature to one controlled by the simultaneous generation of electrons and doubly ionized oxygen vacancies at low pO 2 and higher temperature is identified. At even lower pO 2 , or equivalently larger deviations from stoichiometry, evidence is presented for a further transition to singly ionized oxygen vacancies accompanying electron generation. Temperature induced conductivity relaxation measurements are successfully applied in deconvoluting electron generation and migration contributions to the activation energy. Key parameters are extracted including the enthalpy of reduction H r of 2.87 ± 0.06 eV and the electron hopping or migration energy of 0.354 ± 0.005 eV. Both the activated electron mobility and the broad maximum in conductivity observed under the most reducing conditions support the small polaron model for electron transport in Ce 0.8 Zr 0.2 O 2−δ . Consistent with earlier findings, Zr, though isovalent with Ce, markedly enhances the reducibility, and thereby the oxygen storage capability of ceria− zirconia solid solutions.

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“…This may be related to the observed drop in P O2 sensitivity reported by Tuller and Nowick for highly nonstoichiometric CeO 2 , related to both the hopping mobility of electrons and increasing defect interactions at high values of nonstoichiometry, which become more evident as temperature decreases . Similar findings were recently reported for Ce 0.8 Zr 0.2 O 2‐δ …”

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“…Two peaks of gas consumption exhibited in TPR curve of pristine CeO 2 ( Fig. 4a): one peak at lower temperature (400-600°C) related to the reduction of surface oxygen species and the other at higher temperature (≥700°C) corresponding to the reduction of bulk oxygen [22,23]. In this case, while the H 2 consumption at high temperature is similar for these two samples, we noticed an obvious decrease in the lowtemperature reduction in TPR curve of LT CeO 2 .…”

Electrophoretic deposition of ZnO–CeO2 mixed oxide nanoparticles

“…modeled the electrical conductivity of Ce 0.8 Zr 0.2 O 2−δ . Based on a chemical model, they concluded that this enhanced the reducibility and the oxygen storage capability of ceria‐zirconia . According to Rupp et al.…”

“…2 Based on a chemical model, they concluded that this enhanced the reducibility and the oxygen storage capability of ceria-zirconia. 3 According to Rupp et al who studied time-temperature-transformation (TTT) diagrams of metal oxide thin films, doped ceria films shift the TTT diagrams to longer time and higher temperatures than undoped ceria. 4 Yanuo Shi et al considered ceria microsolid oxide fuel cells and demonstrated that the conductivity of oxygen ionic comparing thin-film ceria-based membrane devices correlates with the compressive strain energy acting on the near of the electrolyte lattices.…”