The best composition of an Y-doped BaZrO3 electrolyte: selection criteria from transport properties, microstructure, and phase behavior

Y-doped BaZrO3 shows high protonic conductivity at intermediate temperature and has promising prospects to be applied as an electrolyte in electrochemical devices, including fuel cells and electrolysis cells. In this work, with the aim to determine the optimal composition of BZY for such electrochemical application, a thorough investigation on BaZrO3 doped with 10-25 mol% Y with an interval of 1 mol% was performed, and the microstructure, phase behavior and transport properties were studied. The results revealed that after sintering at 1600 degrees C for 24 h, a bimodal microstructure was confirmed for BaZr1-xYxO3-delta (x = 0.10-0.15), whereas BaZr1-xYxO3-delta (x = 0.16-0.25) shows a quite uniform grain size. Further investigation on BZY10, BZY15 and BZY20 by sintering for different time periods shows that a uniform grain size of around 2 and 6 mm was obtained for BZY15 and BZY20, respectively, but BZY10 still has a bimodal microstructure, even after sintering at 1600 degrees C for 200 h. Then through a systematic evaluation of the transport properties, we found that when the Y content is above 15 mol%, a total conductivity of over 0.01 S cm(-1) is achieved even at 500 degrees C. And the transport number of ionic conduction in wet oxygen increases with the Y content increasing from 10 to 20 mol%, but does not further increase when the Y content is higher than 20 mol%. Notably, both Ba-deficiency and Ba-excess decrease the transport numbers of ionic conduction in wet oxygen. In conclusion, a Y content of around 20 mol% with strictly controlled Ba stoichiometry seems to be the optimal composition.