Proton, Hydroxide Ion, and Oxide Ion Affinities of Closed-Shell Oxides: Importance for the Hydration Reaction and Correlation to Electronic Structure

Phenomenologically, the enthalpy of the dissociative water incorporation (hydration) of oxides is often found to be more favorable for more basic oxides. In the present work, we investigate proton, hydroxide ion, and oxide ion affinities (PA, HA, and OA) for 19 closed-shell oxides ranging from Li₂O and Cs₂O to TiO₂, SnO₂, and SiO₂, including also perovskites such as SrTiO₃ and BaZrO₃ using first-principles defect calculations and thermochemical cycles. The proton affinity is found to play a predominant role in the hydration thermodynamics. The ion affinities are strongly correlated with the oxides' electronic structure (specifically, the ionization potential (IP)). This intriguing correlation between PA and IP holds also for gaseous O species, suggesting a very general origin. Understanding the major factors controlling a metal oxide's susceptibility for dissociative hydration of oxygen vacancies is not only of fundamental interest but also key to the successful development of novel mixed proton-electron conducting oxides for protonic ceramic fuel and electrolyzer cells. In addition to elucidating the hydration reaction, these ion affinities also serve a more general purpose, as they can be used to predict the oxides' tendency to in-/excorporate a specific ion.