Mechanistic Insights into Hydride Incorporation in BaZr_1–xIn_xO_3−δ-Based Perovskite Oxyhydrides

Metal oxyhydrides have emerged as promising candidates for next-generation electrochemical devices due to their unique combination of hydride (H – ) ion conductivity and catalytic activity for carbon dioxide and nitrogen reduction. However, their limited stability under ambient hydrogen pressure ( p H2 ) poses a significant challenge. In this study, we demonstrated that BaZr 1– x In x O 3–0.5 x (BZI) undergoes topochemical transformation into a thermodynamically stable oxyhydride phase under ambient p H2 across a broad composition range. Indium-rich compositions ( x = 0.5, 0.6, and 0.7) successfully formed oxyhydrides─BaZr 0.5 In 0.5 O 2.25 H 0.5, BaZr 0.4 In 0.6 O 2.18 H 0.23, and BaZr 0.3 In 0.7 O 2.17 H 0.11 ─via the sequential formation of oxygen vacancy (V O ) and subsequent H – ion incorporation. Conversely, the Zirconium-rich composition ( x = 0.3) formed only V O and stabilized as an oxygen-deficient phase, BaZr 0.7 In 0.3 O 2.70, without hydride uptake. Combined experimental and density functional theory (DFT) analyses reveal that the extent of In reduction, reflected by an increase of the Fermi level following V O donor formation, governs the thermodynamic driving force for H – incorporation. DFT results show that a greater initial Fermi energy rise facilitates greater H – uptake, which subsequently lowers Fermi energy upon H – incorporation, establishing an equilibrium determined by defect formation energetics. Furthermore, DFT and in situ X-ray absorption spectroscopy confirm that the oxyhydride phase preferentially forms neutral oxygen vacancies (V O × ) during dehydrogenation, in contrast to the commonly assumed doubly charged species (V O •• ). This reversible transformation between a pure oxyhydride and an oxyhydride–electride phase, dependent on p H2, highlights the tunable defect chemistry and electronic structure of perovskite oxyhydrides and offers new design principles for stable and fast hydride ion conductors.