Defect and Transport Model of Ceria-Zirconia Solid Solutions: Ce0.8Zr0.2O2-δ-An Electrical Conductivity Study

The electrical conductivity (sigma) of Ce0.8Zr0.2O2-delta 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 sigma 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 Hr 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 Ce0.8Zr0.2O2-delta. Consistent with earlier findings, Zr, though isovalent with Ce, markedly enhances the reducibility, and thereby the oxygen storage capability of ceriazirconia solid solutions.