Protonic–electronic mixed-conducting perovskites are relevant as cathode materials for protonic ceramic fuel cells (PCFCs). In the present study, the relation between the electronic structure and the thermodynamics of oxygen nonstoichiometry and hydration is investigated for BaFeO3−δ and Ba0.5Sr0.5FeO3−δ by means of density functional theory. The calculations are performed at the PBE + U level and yield ground-state electronic structures dominated by an oxygen-to-metal charge transfer with electron holes in the O 2p valence bands. Oxygen nonstoichiometry is modeled for 0 ≤ δ ≤ 0.5 with oxygen vacancies in doubly positive charge states. The energy to form an oxygen vacancy is found to increase upon reduction, i.e., decreasing concentration of ligand holes. The higher vacancy formation energy in reduced (Ba,Sr)­FeO3−δ is attributed to a higher Fermi level at which electrons remaining in the lattice from the removed oxide ions have to be accommodated. The energy for dissociative H2O absorption into oxygen vacancies is found to vary considerably with δ, ranging from ≈−0.2 to ≈−1.0 eV in BaFeO3−δ and from ≈0.2 to ≈−0.6 eV in Ba0.5Sr0.5FeO3−δ. This dependence is assigned to the annihilation of ligand holes during oxygen release, which leads to an increase in the ionic charge of the remaining lattice oxide ions. The present study provides sound evidence that p-type electronic conductivity and the susceptibility for H2O absorption are antagonistic properties since both depend in opposite directions on the concentration of ligand holes. The reported trends regarding oxygenation and hydration energies are in line with the experimental observations.

Ab initio calculations based on density functional theory (DFT) were used to study the energetics, geometry of fully relaxed structure, and electronic charge redistribution for adsorbed atomic and molecular oxygen on defectless unreconstructed SrTiO 3 (001) surfaces, both SrO-and TiO 2 -terminated. B3PW functional used in our calculations contains a ''hybrid'' of the DFT exchange and correlation functionals with exact non-local Hartree-Fock (HF) exchange. We performed calculations of two-dimensional slabs with unit cells large enough for the adsorbed species to be treated as isolated. We found substantial binding energies for atomic O adsorption at bridge positions between the two adjacent metal and oxygen surface ions (much closer to the latter) on both SrO-and TiO 2 -terminated surface (over 2.0 eV with respect to free atom). In both cases strong bonding is rather caused by formation of surface molecular peroxide ion in singlet state. For molecular adsorption, different adsorption sites and orientations of O 2 molecule were studied, however, adsorption energy never exceeded 0.1 eV. However, energy gain obtained from adsorption of atomic oxygen is not sufficient for molecule dissociation.

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The proton migration in the triple conducting perovskite BaFeO3-δ is is investigated using first-principles density functional theory calculations. Oxygen-deficient BaFeO3-δ exhibits pronounced lattice distortions that entail different chemical environments of...