Improving NiSe2 HER performance by controlling crystallinity and stabilizing embedded selenium

Nickel diselenide (NiSe 2 ), a promising catalyst for hydrogen evolution reactions (HERs), can be synthesized through a simple chemical reduction method. However, low-temperature synthesis produces secondary crystalline phases, such as Ni 1.7 Se 2 , which diminish catalytic efficiency. This study introduces a post-reduction method to create a stable NiSe 2 /Se heterostructure and examines how crystal structure and selenium content affect electrocatalytic properties. The synthesized products were washed with chloroform and thermally annealed, resulting in NiSe 2 /Se heterostructure crystals. The catalytic performance was assessed at 10 mA/cm 2 , achieving an overpotential of 202 mV and a Tafel slope of 25.3 mV/dec. The enhanced performance is linked to the high-order crystal of NiSe 2 and the presence of selenium, as confirmed by XRD, SAED-TEM, and Raman spectroscopy. The selenium improves carrier densities and charge transfer efficiency. Furthermore, NiSe 2 /Se with lower selenium content retained their initial performance over 21 hours which highlights their stability under HER conditions. • NiSe 2 with Se trapped in the grains was found after annealing. • Optimal Se content increased the carrier densities and charge transfer efficiency. • Crystal structure optimization showed cubic NiSe 2 as the active phase. • Amorphization by chloroform rinse is key to prepare NiSe x- chloro. • An overpotential of 202 mV with Tafel slope of 25.3 mV/dec was found. • Long-term stability test revealed no current density change.