Molecular Level Heterojunction with Sulfur Vacancy of Stable Polyhedral Star Configuration for Boosting Hydroxide Ion Storage

Abstract

The sluggish diffusion of electrons/OH⁻ and poor structural stability restrict the OH⁻ reaction kinetic of metal sulfides for supercapacitors. Herein, a molecular level NiS/Co₉S₈ heterojunction with sulfur vacancy (S_V) and nitrogen-doped carbon (NC) polyhedral star configuration composites (PS-NiS/Co₉S₈@NC) was derived from co-precipitated metal-organic framework (MOF) via in situ ion competitive vulcanization and carbonization strategies. Experiment and theoretical calculations show that the polyhedral star nanostructure with heterojunction exposes more active sites, while the triangular structure covered with NC layer in PS-NiS/Co₉S₈@NC composite plays a favorable supporting function for durable OH⁻ storage. The NiS/Co₉S₈ heterojunction, S_V, and NC coatings synergically optimize the electronic environment and enhance the conductivity. More importantly, the charge redistribution that occurs at NiS/Co₉S₈ can induce a built-in electric field, significantly reducing the OH⁻ diffusion energy barrier and boosting the migration kinetics of electrons/OH⁻. The prepared PS-NiS/Co₉S₈@NC exhibits high reversible capacitance (1902 F g⁻¹ at 1 A g⁻¹), excellent rate capacitance (1212 F g⁻¹ at 30 A g⁻¹), and reliable cycle stability (80.1% retention after 7000 cycles). The assembled hybrid device displays an outstanding energy/power output (54.3 Wh kg⁻¹ and 12706.9 W kg⁻¹). Our work provides a promising way to reasonable design between the structure and function for stable OH⁻ storage.