| Summary: | Microsphere-shaped cobalt selenide (Co<sub>0.85</sub>Se) structures were efficiently synthesized via a two-step hydrothermal process. Initially, cobalt hydroxide fluoride (Co(OH)F) microcrystals were prepared using a hydrothermal method. Subsequently, Co<sub>0.85</sub>Se microsphere-like structures were obtained through selenization. Compared to Co(OH)F, the microsphere-like Co<sub>0.85</sub>Se structure exhibited outstanding catalytic activity for the hydrogen evolution reaction (HER) in a 1.0 M KOH solution. Electrocatalytic experiments demonstrated an exceptional HER performance by the Co<sub>0.85</sub>Se microspheres, characterized by a low overpotential of 148 mV and a Tafel slope of 55.7 mV dec<sup>−1</sup>. Furthermore, the Co<sub>0.85</sub>Se electrocatalyst displayed remarkable long-term stability, maintaining its activity for over 24 h. This remarkable performance is attributed to the excellent electrical conductivity of selenides and the highly electroactive sites present in the Co<sub>0.85</sub>Se structure compared to Co(OH)F, emphasizing its promise for advanced electrocatalytic applications.
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