Transport properties of proton conductive Y-doped BaHfO3 and Ca or Sr-substituted Y-doped BaZrO3

An electrolyte in fuel cells requires not only high ionic conductivity, but also high transport numbers of ionic conduction. Although Y-doped BaZrO3 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 BaZr0.8Y0.2O3-. The results revealed that a single cubic perovskite phase was obtained for Ba0.95Ca0.05Zr0.8Y0.2O3- and Ba1-xSrxZr0.8Y0.2O3- (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, Ba0.95Ca0.05Zr0.8Y0.2O3- shows no meaningful change in the transport number of ionic conduction, compared with BaZr0.8Y0.2O3-. Incorporating Ca or Sr into the Ba-site of BaZr0.8Y0.2O3- 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. BaHfO3 doped with 10 and 20 mol% Y was also prepared. A bimodal microstructure was observed for BaHf0.9Y0.1O3-, whereas BaHf0.8Y0.2O3- shows uniform grain size after sintering at 1600 degrees C for 24 hours. The transport numbers of ionic conduction and bulk conductivity in such Y-doped BaHfO3 samples are close to those of BaZrO3 doped with the same amount of Y.