Guidelines for improving resistance to CO2 of materials for solid state electrochemical systems

Alkali earth oxides are present in many ionic and mixed conductors with a potential application in solid state electrochemical or electroceramic devices. Hence, study has been made of the relative risks of carbonation of selective materials based on thermodynamic predictions, with emphasis on ATiO₃ electroceramics, with A = Ca, Sr, Ba, and proton conductors based on AMO₃ perovskites with A = Sr or Ba and M = Ce or Zr. Predictions show that slight A-site deficiency lowers the activity of alkali earth oxide and enhances resistance to CO₂; this is confirmed by experimental findings. Thermodynamic predictions also indicate that the high stability of (Ba,Sr)TiO₃ and (Ba,Sr)ZrO₃ perovskites contributes to attain reasonable resistance to CO₂, in spite of the ready carbonation of BaO and SrO. However, these materials show wide gap between the upper and lower limits of resistance to CO₂. This gap provides interpretation for the effects of powder preparation and/or ceramic processing on resistance to CO₂. It is also shown that the lower and upper limits of resistance to CO₂ correlate with the structural tolerance factor of these perovskites.