Fermi-Level-Dependent Defect Chemistry and Oxygen Evolution Reaction Activity of Fe-Doped and Oxygen-Deficient \ce{SrTiO3}(001)
The oxygen evolution reaction (OER) on perovskite oxides is controlled by the interplay of dopant chemistry, defect charge states, and surface segregation, yet these factors are rarely treated on equal footing. Using first-principles density functional theory, we investigate how Fe dopants (\FeTi{}) and oxygen vacancies (\VO{}) in different charge states affect the OER on TiO$_2$-terminated \ce{SrTiO3}(001). We combine charge-dependent defect formation energies, segregation energies, and charge transition levels with OER free-energy profiles obtained in the computational hydrogen electrode framework. Neutral \FeTix{} preserves near-pristine activity, with overpotentials of $0.43$--$0.48$~V compared to $0.45$~V for the pristine surface, whereas the reduced states \FeTip{} and \FeTipp{} raise the overpotential to as much as $1.35$~V when intermediates bind to Ti sites adjacent to surface Fe. Oxygen vacancies segregate to the surface across the entire band gap ($\Delta E_\mathrm{seg} = -0.50$ to $-0.80$~eV) but do not improve the activity: \VOx{} and \VOp{} overstabilize oxygenated intermediates ($\eta$ up to $2.13$~V), and only \VOpp{} retains a balanced pathway ($\eta = 0.45$~V in the bulk-like region). Because the stable charge state and the segregation tendency of each defect are set by the Fermi level, the OER overpotential itself becomes a Fermi-level-dependent quantity. These results establish Fermi-level engineering as a framework for assessing and tuning defect-mediated OER activity in perovskite oxides.