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 (H 2 ) to form hydrides, not directly with protons (H + ) in solution. This necessitates an indirect route through the production and compression of high‐purity H 2 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 H 2 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 |Δ P eq | > Δ P ph , 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‐TiH 2 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.