Hydrogen-Trapping Cage Engineering Enables Mild-Condition, Direct Proton Conversion to Functional Metal Hydrides.

The synthesis of metal hydrides is constrained by a fundamental paradigm: metals are believed to react only with molecular hydrogen (H2) to form hydrides, not directly with protons (H+) in solution. This necessitates an indirect route through the production and compression of high-purity H2 gas. We present an alternative synthetic route by repurposing acidic corrosion and hydrogen embrittlement to engineer an in situ hydrogen-trapping cage (HTC) within metals. The HTC enables direct proton‑to‑hydride conversion under mild conditions (ambient pressure, ∼70 % lower temperature). By using protons directly from acids as the hydrogen source, this approach bypasses the need for high-pressure H2 gas, enabling hydride synthesis at ambient pressure and substantially lower temperatures. The process simultaneously constructs a defect-rich microstructure in situ, facilitating rapid ion transport. Guided by the universal criterion |ΔPeq| > ΔPph, we demonstrate the versatility of our method by synthesizing a library of over 20 hydrides, including LiH and NaH; its functional power is exemplified by a cage-rich HTC-TiH2 electrocatalyst, which achieves a nitrate-to-ammonia current density of 1.07 A cm-2 via enhanced H- mobility. This work demonstrates a strategy that couples hydrogen capture, stabilization, and conversion within a single material system, providing a potential route for sustainable hydrogen management.