Phase Transition in Cobalt Selenide with a Greatly Improved Electrocatalytic Activity in Hydrogen Evolution Reactions

Surface properties such as electronic structures and valence state determine the electrocatalytic activity in hydrogen evolution reactions (HERs). Herein, we prepared a novel orthorhombic CoSe2-NC (o-CoSe2-NC) electrocatalyst from hexagonal CoSe-NC (h-CoSe-NC) via a phase-transition method. As-prepared o-CoSe2-NC exhibited excellent HER activity with 147 mV at 10 mA cm–2 and a high stability (24 h). Density functional theory results revealed that the active sites for h-CoSe-NC in electrocatalytic hydrogen evolutions were the anion Se sites (ΔGH* = 0.34 eV). After the phase transition, Co sites (ΔGH* = 0.20 eV) in o-CoSe2-NC became more active than Se sites. Moreover, the d-band central of Co in o-CoSe2-NC was closer to the Fermi level than that of h-CoSe-NC after the phase transition. Also, o-CoSe2-NC exhibited a metallic behavior with excellent electrical conductivity. This work not only prepared cost-effective cobalt selenide-based electrocatalysts but also highlighted the significance of the crystal phase in electrocatalytic hydrogen evolutions.