Basicity and hydroxyl capacity of proton-conducting perovskites

In order to understand the large solubility of protons in perovskite proton conductors, a new approach employing the hydroxyl capacity, termed COH′≡[OHO·]·[VO··]−1·pH2O−1/2, is proposed and examined for 5 mol% YbO1.5-doped BaCeO3. Prior to the solubility experiments, the phase relation in an isothermal section of the phase diagram for BaO–CeO2–YbO1.5 was determined and activity measurements for BaO were carried out. The values of COH′ were evaluated by measuring the solubility of water using the thermogravimetric method under a controlled BaO activity by employing the reaction BaO(in BaCeO3)+CO2(g)=BaCO3. COH′ did not exhibit a strong dependency on the BaO activity, indicating that the basicity of BaCeO3, defined as the activity of O2− ions, is almost constant across the homogeneity region. It has been estimated that the A-site vacancy in ABO3 mainly contributes to the steep change in BaO activity.