Optimizing proton conductivity in alkaline-earth zirconates through defect engineering

The alkaline-earth zirconates (AeZrO₃; Ae = Sr, Ca, Ba) are among the best solid-state proton conductors and as such have energy applications in solid-state hydrogen fuel cells. They crystallize as perovskite oxides of the form ABO₃. Oxygen vacancies form readily in these systems, and upon exposure to water, these vacancies are filled and the materials are populated with protons. To incorporate oxygen vacancies, acceptor dopants such as Sc and Y substituting on the Zr-site are used; however, these dopants can simultaneously incorporate as A-site donors and self-compensate. We study the properties of these dopants using first-principles calculations with a hybrid functional. We characterize the bulk properties and study the formation of native and extrinsic point defects. We examine the propensity for self-compensation of Sc and Y, as well as possible A-site and O-site acceptors. We find that certain alkali metal A-site dopants (Na in CaZrO₃, K in SrZrO₃, and Rb in BaZrO₃) incorporate oxygen vacancies in higher concentrations than Sc and Y, while also circumventing the problem of self-compensation. Furthermore, these acceptors have low proton binding energies, making them good choices to improve proton conductivity in the zirconates.