Construction of MOF-derived hollow Ni-Zn-Co-S nanosword arrays as binder-free electrodes for asymmetric supercapacitors with high energy density.

Mixed transition metal sulfides with hollow structures hold great promise for energy-related applications. However, most of them are in the powder form, which should be mixed with unwanted polymer binders and conductive agents. In this study, a facile two-step strategy has been developed to grow mesoporous and hollow Ni-Zn-Co-S nanosword arrays (NSAs) on a nickel foam (NF) substrate with robust adhesion, which involves the hydrothermal growth of bimetallic Zn-Co-ZIF NSAs on NF and subsequent transformation into hollow Ni-Zn-Co-S NSAs through the sulfurization process. Benefiting from the unique structural and compositional advantages as well as directly grown conductive substrate, the Ni-Zn-Co-S-0.33 NSAs/NF electrode exhibits the best electrochemical performance when investigated as a binder-free electrode for supercapacitors. Impressively, the Ni-Zn-Co-S-0.33 NSAs/NF electrode delivers a high areal capacity of 1.11 mA h cm at the current density of 10 mA cm, and the corresponding specific capacity is as high as 358.1 mA h g. Moreover, an asymmetric supercapacitor (ASC) device based on the Ni-Zn-Co-S-0.33 NSAs/NF as the positive electrode and BiO/NF as the negative electrode has been successfully fabricated, and can deliver a high energy density of 91.7 W h kg at a power density of 458 W kg and maintain the energy density of 66.9 W h kg at a high power density of 6696 W kg. The electrochemical results suggest that the hollow Ni-Zn-Co-S NSAs would possess great potential for applications in high-performance supercapacitors.