Combining two redox active rare earth elements for oxygen storage – electrical properties and defect chemistry of ceria–praseodymia single crystals

Abstract

Solid solutions of ceria and praseodymia are highly relevant for electrochemical applications as the incorporation of praseodymium into the ceria lattice shifts the range of mixed ionic electronic conductivity to higher oxygen partial pressures. To better understand the influence of praseodymium substitution on the transport processes and oxygen storage capacity in ceria, single crystals of ceria substituted with 14 mol% praseodymium have been investigated, obtaining the bulk properties without the influence of grain boundaries. Beside the characterization of structural changes caused by the substitution using XRD and Raman spectroscopy, the electrochemical transport properties of ceria–praseodymia single crystals are reported. Measurements of the total electrical conductivity, the ionic transference number and the non-stoichiometry of Ce_0.85Pr_0.14Zr_0.01O_2−δ were performed in an oxygen partial pressure range of −25 < lg[p(O₂)/bar] < 0 at 700 °C. With praseodymium being redox active itself, higher values of oxygen deficiency and electrical conductivity than in pure ceria have been observed in the high oxygen partial pressure region, while no significant structural changes occur due to the similar ionic radii of both cations. From measurements of the impedance at different temperatures, the migration enthalpy for the electronic charge carriers has been determined. By analysing the non-stoichiometry at 700 °C using a defect chemical model it was also possible to determine the equilibrium constants of Pr and Ce reduction in Ce_0.85Pr_0.14Zr_0.01O_2−δ single crystals.