Transport properties of proton conductive Y‐doped BaHfO ₃ and Ca or Sr‐substituted Y‐doped BaZrO ₃

An electrolyte in fuel cells requires not only high ionic conductivity, but also high transport numbers of ionic conduction. Although Y‐doped BaZrO 3 is regarded to be the most promising candidate as the electrolyte in protonic ceramic fuel cells (PCFCs), significant hole conduction generates in wet oxygen at high temperatures. With the aim to increase the transport number of ionic conduction, in this work, Sr and Ca were introduced to partially substitute Ba in BaZr 0.8 Y 0.2 O 3‐δ . The results revealed that a single cubic perovskite phase was obtained for Ba 0.95 Ca 0.05 Zr 0.8 Y 0.2 O 3‐δ and Ba 1‐ x Sr x Zr 0.8 Y 0.2 O 3‐δ ( x = 0.05, 0.10, 0.15, 0.20 or 0.40). However, replacing Ba with Sr resulted in almost no increase in the transport number of ionic conduction in wet oxygen atmosphere, but drastic decrease in proton conductivity at all replacement levels. In addition, Ba 0.95 Ca 0.05 Zr 0.8 Y 0.2 O 3‐δ shows no meaningful change in the transport number of ionic conduction, compared with BaZr 0.8 Y 0.2 O 3‐δ . Incorporating Ca or Sr into the Ba‐site of BaZr 0.8 Y 0.2 O 3‐δ appears to impart no positive influence on electrochemical properties. These interesting results also indicate that the hole conductivity decreases with the decrease in proton conductivity, and will aid to consider the hole conduction mechanism. BaHfO 3 doped with 10 and 20 mol% Y was also prepared. A bimodal microstructure was observed for BaHf 0.9 Y 0.1 O 3‐δ , whereas BaHf 0.8 Y 0.2 O 3‐δ shows uniform grain size after sintering at 1600°C for 24 hours. The transport numbers of ionic conduction and bulk conductivity in such Y‐doped BaHfO 3 samples are close to those of BaZrO 3 doped with the same amount of Y.