Design of trimetallic sulfide hollow nanocages from metal-organic frameworks as electrode materials for supercapacitors.

Transition metal sulfides (TMSs) are the most used electrode materials for supercapacitors (SCs). However, they still suffer from unsatisfactory electrochemical properties. Designing a hollow mixed TMS nanostructure with a well-defined chemical composition and shape is an effective strategy to tackle this issue, yet remains challenging. Herein, using a bimetallic zeolitic imidazolate framework (Zn-Co-ZIF) with various Zn/Co ratios as the template, a series of trimetallic sulfide (Ni-Zn-Co-S) hollow nanocages were successfully prepared by sequential nickel nitrate etching, co-precipitation and vulcanization. As an electrode material for a three-electrode SC in an aqueous alkaline electrolyte, the Ni-Zn-Co-S-0.25 electrode achieves an ultra-high specific capacitance of 1930.9 at 1 A g-1 with a good rate performance (64.5% at 10 A g-1). In order to further prove the advantage of the as-prepared Ni-Zn-Co-S-0.25 material, it was assembled into an asymmetric energy storage device using an activated carbon (AC) anode. The Ni-Zn-Co-S-0.25//AC cell exhibits an outstanding energy storage capability (32.8 W h kg-1 at 864.8 W kg-1) with a splendid cyclic life (retaining ∼92.2% of the initial capacitance after 10 000 cycles). The excellent electrochemical performance of Ni-Zn-Co-S-0.25 is ascribed to the merits of the trimetallic sulfide hollow nanocage i.e., good electronic conductivity, a large active surface area, fast charge transfer, rich redox reactions and the synergic effect of different metal ions.