Transport properties of Ba₃Ca_1+xNb_2−xO_9−δ composite perovskite oxides

ABO₃-type perovskite oxides and A₃B′B′′₂O₉-type composite perovskite oxides exhibit proton conduction from 200 ℃ to 1000 ℃. These high-temperature proton conductors have received considerable attention due to their promise as electrolytes in fuel cells, electrolytic hydrogen production, hydrogen separation, electrochemical reactors, sensors, etc. The Ba₃Ca_1+xNb_2−xO_9−δ composite perovskite-type solid electrolyte has stable chemical properties and corrosion resistance to CO₂ and H₂O, so it can be used in long-term electrochemical devices. Protons are incorporated into Ba₃Ca_1+xNb_2−xO_9−δ in a humid or hydrogen-containing atmosphere because of the reaction of H₂O and oxygen vacancies in proton conductors. However, proton conductors also exhibit oxygen vacancy conduction in the high-temperature range. In addition, electron holes can be generated by an oxygen vacancy reaction with atmospheric oxygen, causing proton conductors to exhibit electron-hole conduction. Hence, more oxygen vacancies can be produced with more Ca²⁺ dopant in Ba₃Ca_1+xNb_2−xO_9−δ due to a lack of positive charge. Meanwhile, the proton and electron-hole concentrations increase with oxygen vacancies, and the conductivity of Ba₃Ca_1+xNb_2−xO_9−δ can be improved. However, the crystal structure of Ba₃Ca_1+xNb_2−xO_9−δ can be changed with Ca²⁺ doping, and changes in proton, oxygen vacancy, and electron-hole transport numbers, the ratio of protons, oxygen vacancies, and electron-hole conductivity to total conductivity respectively, are unknown with Ca²⁺ doping, with different effects of crystal structure for protons, oxygen vacancies, and electron-hole conduction. Ba₃Ca_1+xNb_2−xO_9−δ has high conductivity in a humid atmosphere, and the proton transport number with doping amount needs to be further studied. In this work, Ba₃Ca_1+xNb_2−xO_9−δ (x=0, 0.10, 0.18, and 0.30) with a composite perovskite phase was prepared using a solid-state reaction method. With the increase in Ca²⁺ doping amount, the conductivity of Ba₃Ca_1+xNb_2−xO_9−δ samples first increased and then decreased, and the conductivity of the sample with x=0.18 was the highest. The electron-hole transport number of Ba₃Ca_1+xNb_2−xO_9−δ under the atmosphere containing hydrogen was relatively low. Protons were mainly conductive carriers in Ba₃Ca_1+xNb_2−xO_9−δ below 750 ℃, while Ba₃Ca_1+xNb_2−xO_9−δ exhibited mainly oxygen vacancy conduction at 800 ℃. With the increase in dopant amount, the oxygen vacancy transport number of Ba₃Ca_1+xNb_2−xO_9−δ increased gradually, while the proton transport number decreased gradually.