Atmosphere- and voltage- dependent electronic conductivity of oxide ion conducting Zr1-xYxO2-x/2 ceramics

Yttria-stabilised zirconia, YSZ, is the most widely-used oxide ion conductor in solid oxide fuel cells, SOFCs. The key to this application is the high concentration of mobile anion vacancies created to charge-compensate for... [ view full abstract ]

Yttria-stabilised zirconia, YSZ, is the most widely-used oxide ion conductor in solid oxide fuel cells, SOFCs. The key to this application is the high concentration of mobile anion vacancies created to charge-compensate for introduce Y³⁺ dopant onto the Zr⁴⁺ sites. YSZ is a solid solution series of general formula Zr_1-xY_xO_2-x/2 which has the fluorite structure in which the concentration of oxygen vacancies depends on the value of x. The composition most widely used in SOFC applications with highest oxide ion conductivity is x=0.08, YSZ08.

For all oxide ion conducting solid electrolytes, the electrolytic domain is limited at low oxygen partial pressures, pO₂, by the onset of n-type electronic conduction and at high pO₂ by the onset of p-type conduction. Recently, electronic conduction has been introduced into YSZ08 under the action of a small dc bias at high temperature in air. The explanation was based on similar conductivity changes seen with acceptor-doped titanate perovskites and Ca-doped BiFeO₃, which all show enhanced conductivity on either increasing pO₂ in the measuring atmosphere or application of a small dc bias. The enhanced conductivity was attributed to ionization of underbonded O^2- ions in the vicinity of acceptor dopants leading to hole creation on oxygen which conceptually, is the same as the creation of O^- ions. However, the conductivity of YSZ with x = 0.08 is not sensitive to pO₂ in atmospheres such as O₂, N₂ or air. Preliminary tests on compositions with higher x showed that their conductivity is indeed, sensitive.

 The present work reports a systematic study of the conductivity of solid solutions Zr_1-xY_xO_2-x/2, with 0.42 and dc bias on the electrical properties. The effects of increasing pO₂ and application of a dc bias are similar in kind, and results showed a significant increase in electronic conductivity.