Central Role of Transition-Metal Electronic Structure in Competing Hydrogenation and Oxygen-Vacancy Formation in SrMOy.

The emergence of hydrogen (H) insertion as a powerful strategy for tailoring the structure and functionality of transition-metal oxides (TMOs) has enabled applications ranging from protonic electronics and neuromorphic devices to energy conversion and storage. However, the fundamental electronic-structure factors governing hydrogen uptake and the competing formation of oxygen vacancies (VO••) remain poorly understood, limiting the rational design of hydrogenated oxides. In this study, we combined density functional theory, machine learning-enhanced global optimization with first-principles energy expressions, and thermodynamic analysis to investigate H insertion and topotactic phase transitions in SrMOy (M = Fe, Co; 2.0 ≤ y ≤ 3) with further examination of other 3d transition-metal ions (V, Cr, Mn, and Ni). Our key finding revealed a crucial role of the transition-metal electronic structure in determining the reaction pathway upon H insertion. Oxide reduction via the VO•• formation is favored when the electron donated by H populates the eg states, which strongly weakens M-O bonds through occupation of pdσ∗ antibonding states. By contrast, oxide hydrogenation is favored when the electron is accommodated in the t2g states with weaker pdπ∗ antibonding character. SrMnO2.5 and SrFeO2.5 represented notable exceptions. The H insertion in SrMnO2.5 stabilizes high-spin Mn2+ (d5) without requiring strongly distorted tetrahedral or planar MnO4 coordination associated with the VO•• formation, while the large exchange splitting in SrFeO2.5 raises the energy of the electron-accepting states and limits H insertion. These findings establish the mechanism by which the electronic structure of transition metals governs the competition between hydrogenation and VO••formation in TMOs.