The doping improves the catalytic activity through (1) increasing oxygen vacancy concentration by 87.5 and 65.5% at room temperature and 800 °C, respectively, as demonstrated by iodometric titration and thermogravimetric analysis, (2) greatly reducing the energy for oxygen vacancy formation as shown by density functional theory calculation, (3) forming additional reactive oxygen species at the near surface region as suggested with X-ray photoelectron spectroscopy, and (4) enhancing the oxygen transport properties as exhibited with electrical conductivity relaxation.  In addition, bismuth doping reduces the thermal expansion coefficient to a level that could exactly match the thermal expansion behavior to the electrolytes. 
THERM