Temperature-Induced Structure Transformation from Co 0.85 Se to Orthorhombic Phase CoSe 2 Realizing Enhanced Hydrogen Evolution Catalysis.

Transition-metal chalcogenides (TMC) have been widely studied as active electrocatalysts toward the hydrogen evolution reaction due to their suitable d-electron configuration and relatively high electrical conductivity. Herein, we develop a feasible method to synthesize an orthorhombic phase of CoSe 2 (o-CoSe 2 ) from the regeneration of Co 0.85 Se, where the temperature plays a key role in controlling the structure transformation. To the best of our knowledge, this is the first report about this synthetic route for o-CoSe 2 . The resulting o-CoSe 2 catalysts exhibit enhanced hydrogen evolution reaction performance with an overpotential of 220 mV to reach 10 mA cm -2 in 1.0 M KOH. Density functional theory calculations further reveal that the change in the Gibbs free energy of hydrogen, water adsorption energy, and the downshifted d-band center make o-CoSe 2 more suitable for accelerating the HER process.