First-Principles Insight into the Effects of Intrinsic Oxygen Defects on Proton Conduction in Ruddlesden-Popper Oxides.

Understanding proton transport in Ruddlesden-Popper (RP) oxides, as attractive electrode materials for protonic ceramic fuel cells, is challenging because of the complexity of intrinsic oxygen defects in first-series RP oxides (A2BO4). We investigated the processes of intrinsic oxygen defects in proton transportation, such as formation of defects, incorporation of dissociative water into the defective lattice, transfer of a proton along the oxygen sites, and electronic properties of the transition state (TS) in A2BO4. The coexistence of oxygen vacancies (VO) and interstitial oxygen (Oi), VO+Oi defect pair, presents advantageous hydration energies and lattice distortions efficiently accelerating proton transport in the lattice. Moreover, the inherent driving force for proton transport is related to the O 2p band level by O-H···O bond interactions in the TS. Our findings elucidate the fundamental mechanism of proton conduction affected by intrinsic oxygen defects, which will motivate the community to focus more on defect engineering to enhance performance.