A Thermodynamic Framework for Predicting Oxygen Vacancy Formation Energies on Electrocatalyst Surfaces

Oxygen vacancies (V_O) are widely invoked as key descriptors of the activity of transition metal oxide electrocatalysts. However, their thermodynamic stability under realistic electrochemical operating conditions remains poorly understood. In this work, we establish a thermodynamic framework for calculating V_O formation energies as a function of pH and applied potential. Through a systematic evaluation of 19 representative oxygen reduction reaction (ORR) and evolution reaction (OER) catalysts, we demonstrate that while V_O formation can be energetically favorable at standard states (0 V vs RHE), the formation energies increase significantly under operando conditions, exceeding 0.50 eV for ORR and 2.00 eV for OER. These large thermodynamic barriers suggest that surface oxygen vacancies are unlikely to persist as stable, long-lived surface defects during electrocatalysis, particularly under the oxygen-rich OER conditions. Our findings challenge the prevailing assumption of vacancy-mediated reaction mechanisms and highlight the necessity of accounting for potential-dependent surface stoichiometry when modeling the catalyst-electrolyte interface.