Formation and Migration of Oxygen Vacancies in SrCoO₃ and Their Effect on Oxygen Evolution Reactions

Abstract

Perovskite SrCoO 3 is a potentially useful material for promoting the electrocatalytic oxygen evolution reaction, with high activities predicted theoretically and observed experimentally for closely related doped perovskite materials. However, complete stoichiometric oxidation is very difficult to realize experimentally -in almost all cases there are significant fractions of oxygen vacancies present. Here, using first principles calculations we study oxygen vacancies in perovskite SrCoO 3 from thermodynamic, electronic and kinetic points of view. We find that an oxygen vacancy donates two electrons to neighboring Co sites in the form of localized charge. The formation energy of a single vacancy is very low and estimated to be 1.26 eV in the dilute limit. We find that a vacancy is quite mobile with a migration energy of ~0.5 eV. Moreover, we predict that oxygen vacancies exhibit a tendency towards clustering which is in accordance with the material's ability to form a variety of oxygen-deficient structures. These vacancies have a profound effect on the material's ability to facilitate OER, increasing the overpotential from ~0.3 V for the perfect material to ~0.7 for defective surfaces. A moderate compressive biaxial strain (2%) is predicted here to increase the surface oxygen vacancy formation energy by ca. 30%, thus reducing the concentration of surface vacancies and thereby preserving the OER activity of the material.

Sc and Nb dopants in SrCoO3 modulate electronic and vacancy structures for improved water splitting and SOFC cathodes

“…This differs from the undoped case of SrCoO 3 where the two electrons tend to localize on neighbouring Co sites. [35] It has been shown that the vacancy formation energy correlates with the p-band centre, such that either of which could be used as an activity descriptor [16,52]. This correlation also holds for the structures considered here.…”

“…The increased oxygen vacancy formation energy for low Sc and Nb doping fractions maintains a near stoichiometric concentration of oxygen ions. Our earlier work has revealed a detrimental role of surface oxygen vacancies on the OER activity in SrCoO 3 [35]. Hence, minimizing oxygen vacancy concentrations can be expected to help minimize the overpotential for OER.…”

“…Based on earlier work, this supercell size was shown to be large enough to model a vacancy in the dilute limit. [35] …”

“…[36] Starting from low Sc doping fractions (x = 0.037), oxygen vacancies adjacent to the dopant atoms have high formation energies relative to a single isolated vacancy in SrCoO 3 [35] which was found to be 1.26 eV in the dilute limit (1.37 eV in a 3x3x3 supercell). For instance the Nb-V O -Co forms at 1.9 eV whereas Sc-V O -Co forms at 1.54 eV.…”

Engineering the oxygen coordination in digital superlattices

“…Furthermore, we demonstrate control over the spatial arrangement of oxygen vacancies in an oxide crystal, which we and others have shown to be useful in modulating electronic, magnetic, and catalytic behaviors. [36][37][38][39] By employing different arrangements of CoO x and TiO 2 layers under the same epitaxial strain and growth conditions, the Co oxidation state can be raised from the nominal +3 found in brownmillerite SrCoO 2.5 to +3.25 and +3.5 in (SrCoO 2.64 ) 2 : (SrTiO 3 ) 1 and (SrCoO 2.76 ) 1 : (SrTiO 3 ) 1 superlattices, respectively. The results illustrate how interface engineering can be combined with defect control to achieve novel properties in complex oxides.…”