A double-confined strategy for enhancing the pseudocapacitance performance of nickel-based sulfides-unveiling aqueous pseudocapacitive energy storage mechanism.

Lower voltage window and limited pseudocapacitive active sites are identified as critical impediments hindering the advancement of nickel-based supercapacitors. Herein, a double-confined strategy involving nanosizing and heterointerfaces is proposed to construct nickel-based sulfide composite (PMO@NiS₂/Ni_0.96S@C) with abundant oxygen vacancies (O_V), sulfur vacancies (S_V), and heterostructures. The composite was prepared using liquid-phase in situ self-assembly and low-temperature in situ induction techniques. The double-confined structure and the introduction of vacancies can effectively expose the pseudocapacitive active sites and improve the operating voltage window range of nickel nanosulfides to enhance pseudocapacitive performance. It is determined that anions and cations in the electrolyte are collectively implicated in the energy storage process. Meanwhile, electrochemical quasi-in situ XPS, in situ electrochemical quartz crystal microbalance (EQCM), and theoretical calculations based on density functional theory (DFT) were utilized to verify the energy storage mechanisms of anions and cations in the electrolyte. Furthermore, a pseudocapacitive reaction mechanism for the composites is proposed, which encompasses a novel charge storage coupling effect between the surface redox reaction of the electrolyte anions and the intercalation/de-intercalation of the electrolyte cations at the interlayer and heterointerface. Consequently, the (PMO@NiS₂/Ni_0.96S@C) electrode achieves 1807 C/g (6 M KOH, 0.25 A/g) under the working potential window of -0.8 ∼ 0.5 V. The assembled symmetric supercapacitor demonstrates a specific potential of 2 V, yielding an energy density of 96 Wh kg^-1 at a power density of 300 W kg^-1. This work provides a theoretical reference for designing nickel-based compound materials with high energy density.