Symbiotic Ni3Se4/Ni heterostructure induced by unstable NiSe2 for enhanced hydrogen generation

• Ni 3 Se 4 /NiSe 2 will undergoes in-situ reconstruction under alkaline HER condition. • The real catalytic active species of Ni 3 Se 4 /NiSe 2 during alkaline HER is Ni 3 Se 4 /Ni. • The intrinsic structure instability is the internal reason for NiSe 2 reconstruction. • Formation of Ni 3 Se 4 /Ni can optimize the ε d -Ni and expose more active sites. Creating dual-site catalysts that can separately provide good OH and H interaction sites for water dissociation and hydrogen adsorption is proved to be an effective strategy for improving the activity of hydrogen generation in alkaline medium. However, there is still great challenge to construct a robust dual-site catalyst with multiple and efficient interfaces. Herein, taking thermodynamically unstable cubic NiSe 2 phase as a self-sacrificial template, a symbiotic Ni 3 Se 4 /Ni heterostructure is successfully synthesized via in-situ electrochemical reduction of Ni 3 Se 4 /NiSe 2 precatalyst. Density functional theory calculation suggests that the d-band center and electronic state of Ni sites are well optimized after forming the Ni 3 Se 4 /Ni heterostructure, which is favorable for the proper hydrogen adsorption. Benefiting from the good electrical conductivity, large electrochemical surface area, and distinct sites for water dissociation, Ni 3 Se 4 /Ni exhibits a low overpotential of 106 mV at -10 mA cm -2 for hydrogen generation in 1 M KOH.