text: By the addition of yttria to pure zirconia (e.g., fully stabilized YSZ) Y ions replace Zr on the cationic sublattice. Thereby, oxygen vacancies are generated due to charge neutrality:
  meaning that two Y ions generate one vacancy on the anionic sublattice. This facilitates moderate conductivity of yttrium-stabilized zirconia for O ions (and thus electrical conductivity) at elevated and high temperature. This ability to conduct O ions makes yttria-stabilized zirconia well suited for application as solid electrolyte in solid oxide fuel cells.
  For low dopant concentrations, the ionic conductivity of the stabilized zirconias increases with increasing YO content. It has a maximum around 8–9 mol% almost independent of the temperature (800–1200 °C). Unfortunately, 8–9 mol% YSZ (8YSZ, 8YDZ) also turned out to be situated in the 2-phase field (c+t) of the YSZ phase diagram at these temperatures, which causes the material's decomposition into Y-enriched and depleted regions on the nanometre scale and, consequently, the electrical degradation during operation. The microstructural and chemical changes on the nanometre scale are accompanied by the drastic decrease of the oxygen-ion conductivity of 8YSZ (degradation of 8YSZ) of about 40% at 950 °C within 2500 hours. Traces of impurities like Ni, dissolved in the 8YSZ, e.g., due to fuel-cell fabrication, can have a severe impact on the decomposition rate (acceleration of inherent decomposition of the 8YSZ by orders of magnitude) such that the degradation of conductivity even becomes problematic at low operation temperatures in the range of 500–700 °C.
  Nowadays, more complex ceramics like co-doped zirconia (e.g., with scandia) are in use as solid electrolytes.