Delocalization of d-electrons induced by cation coupling in ultrathin Chevrel-phase NiMo₃S₄ nanosheets for efficient electrochemical water splitting

Chevrel-phase metal sulfides are known to be promising materials for energy conversion and storage applications. However, a detailed understanding of the intrinsic kinetic mechanisms of electrocatalytic bifunctional hydrogen and oxygen evolution reactions (HER/OER) on NiMo₃S₄-based Chevrel-phases is lacking. Herein, novel ultrathin self-assembled nanosheets of NiMo₃S₄ are coupled with transition metal atoms (M/N-NiMo₃S₄; where M = Co, Fe, and Cu) were formed by a facile hydrothermal approach. Notably, the Co/N-NiMo₃S₄ electrocatalyst exhibits excellent performance in terms of ultralow overpotentials of 78, 208, 282, and 307 mV at 10, 100, 500, and 1000 mA cm⁻² for the HER; and 186, 204, and 225 mV at 50, 100, and 300 mA cm⁻² for the OER, respectively. Experimental and first principle calculations demonstrate that Co atoms coupling with edge Ni atoms results in d‐electron delocalization on Co/N-NiMo₃S₄, signifying the efficient charge transfer to improve overall water electrolysis. In addition, an upshift in the d‐band center of Co/N-NiMo₃S₄ can optimize the free energies of a variety of reaction intermediates for water adsorption and dissociation; thereby facilitating the robust alkaline overall water electrolysis at 1.47 V. This work therefore greatly deepens the understanding of the bifunctional hydrogen and oxygen evolution reaction of Chevrel-phase electrocatalysts.