Closest Packing Polymorphism Interfaced Metastable Transition Metal for Efficient Hydrogen Evolution

Metastable materials are promising because of their catalytic properties, high‐energy structure, and unique electronic environment. However, the unstable nature inherited from the metastability hinders further performance improvement and practical applications of these materials. Herein, this limitation is successfully addressed by constructing an in situ polymorphism interface (inf) between the metastable hexagonal‐close‐packed (hcp) phase and its stable counterpart (face‐centered cubic, fcc) in cobalt–nickel (CoNi) alloy. Calculations reveal that the interfacial synergism derived from the hcp and fcc phases lowers the formation energy and enhances stability. Consequently, the optimized CoNi‐inf exhibits an exceptionally low potential of 72 mV at 10 mA cm−2 and a Tafel slope of 57 mV dec−1 for the hydrogen evolution reaction (HER) in 1.0 m KOH. Furthermore, it is superior to most state‐of‐the‐art non‐noble‐metal‐based HER catalysts. No noticeable activity decay or structural changes are observed even over 14 h of catalysis. The computational simulation further rationalizes that the interface of CoNi‐inf with a suitable d‐band center provides uniform sites for hydrogen adsorption, leading to a distinguished HER catalytic activity. This work, therefore, presents a new route for designing metastable catalysts for potential energy conversion. A closest‐packing polymorphism interface is constructed between the metastable hexagonal‐close‐packed phase and its stable counterpart (face‐centered cubic) in a transition‐metal‐based catalyst, promoting catalytic activity and long‐term durability toward the alkaline hydrogen evolution process.