Covalent 0D–2D Heterostructuring of Co 9 S 8 –MoS 2 for Enhanced Hydrogen Evolution in All pH Electrolytes

Abstract Ultrasmall Co 9 S 8 nanoparticles are introduced on the basal plane of MoS 2 to fabricate a covalent 0D–2D heterostructure that enhances the hydrogen evolution reaction (HER) activity of electrochemical water splitting. In the heterostructure, separate phases of Co 9 S 8 and MoS 2 are formed, but they are connected by Co–S–Mo type covalent bonds. The charge redistribution from Co to Mo occurring at the interface enhances the electron‐doped characteristics of MoS 2 to generate electron‐rich Mo atoms. Besides, reductive annealing during the synthesis forms S defects that activates adjacent Mo atoms for further enhanced HER activity as elucidated by the density functional theory (DFT) calculation. Eventually, the covalent Co 9 S 8 –MoS 2 heterostructure shows amplified HER activity as well as stability in all pH electrolytes. The synergistic effect is pronounced when the heterostructure is coupled with a porous Ni foam (NF) support to form Co 9 S 8 –MoS 2 /NF that displays superior performance to those of the state‐of‐the‐art non‐noble metal electrocatalysts, and even outperforms a commercial Pt/C catalyst in a practically meaningful, high current density region in alkaline (>170 mA cm −2 ) and neutral (>60 mA cm −2 ) media. The high HER performance and stability of Co 9 S 8 –MoS 2 heterostructure make it a promising pH universal alternative to expensive Pt‐based electrocatalysts for practical water electrolyzers.