In situ Reconstruction of Nickel Selenide for Enhanced Hydrogen Evolution Reaction

Abstract Transition metal chalcogenides have been widely explored as cost‐effective alternative electrocatalysts to precious metals like Pt for the hydrogen evolution reaction (HER) under alkaline conditions. However, these materials usually experience dynamic reconstruction during HER, and hence identifying the true active species remains elusive, but is crucial for the design of highly active and durable catalysts. Here, single‐crystal NiSe is employed as model catalysts to investigate dynamic reconstruction and true active species under alkaline HER conditions. Ex situ and operando characterizations reveal that NiSe experiences a unique phase transition into Ni ₃ Se ₂ /NiO heterostructure during HER, which is the actual active species. This behavior is distinct from conventional reconstructed active species, which are typically metallic or single oxide or hydroxide phases. Structure analysis reveals a reconstruction mechanism whereby long Ni‐Se bonds and a Se 4p band closer to Fermi level drive NiSe to Ni ₃ Se ₂ transformation, while in Ni ₃ Se ₂ , a 3D Ni‐Ni bond network and high‐energy Ni 3d bands promote surface oxidation to NiO. Further theoretical calculations suggest that electronic interactions at the Ni ₃ Se ₂ /NiO interface accelerate the water dissociation by anchoring OH ^* on Ni sites of NiO and H ^* on Se sites of Ni ₃ Se ₂ , while optimized Se site electronic structure promotes H ₂ generation. The dual‐active sites reduce the overpotential by 80 mV at a current density of 10 mA cm ⁻² compared to pristine single‐crystal NiSe. Remarkably, this concept was validated in a study of nano‐NiSe catalysts that achieved superior performance, with an overpotential of only 70 mV at 10 mA cm ⁻² and excellent stability over 40 h at an overpotential of 400 mV, surpassing most reported NiSe‐based catalysts. This work not only enriches the understanding of HER active species but also establishes reconstruction‐guided design as a powerful methodology for developing high‐performance heterostructure electrocatalysts, demonstrating how fundamental mechanistic insights from model systems can guide practical catalyst.