Electrochemical Charge Storage Behavior of Various Ni Co 2 S 4 Hierarchical Microstructures

The micro- and/or nanostructures of electrode materials play an essential role in their electrochemical performance, and the further understanding of their electrochemical behavior is vitally important for the rational design of high-performance electrochemical energy storage devices (EESDs). Herein, four types of ${\mathrm{Ni}\mathrm{Co}}_{2}{\mathrm{S}}_{4}$ hierarchical microstructures (HMSs; i.e., sphere-, hat-, brush-, and flowerlike HMSs) assembled from similar hollow nanoneedle building blocks are specially synthesized via a multistep strategy. A series of electrochemical analyses are performed to investigate the structure-activity relationship of these ${\mathrm{Ni}\mathrm{Co}}_{2}{\mathrm{S}}_{4}$ electrodes. The four ${\mathrm{Ni}\mathrm{Co}}_{2}{\mathrm{S}}_{4}$ electrodes all exhibit hybrid electrochemical characteristics combined with the adsorption and diffusion-controlled process of Faradaic redox reaction kinetics. Notably, the brushlike ${\mathrm{Ni}\mathrm{Co}}_{2}{\mathrm{S}}_{4}$ electrode displays the best specific capacitance (capacity) of 1684 F g (234 mAh g) at 1 A g, and retains 91% of its initial performance after 10 000 cycles, which is mainly due to its well-ordered three-dimensional microstructure with sufficient surface reaction sites, fast charge transport, and efficient ion diffusion properties. The resulting hybrid supercapacitor delivers a high energy density of 2667 mWh kg with a power density of 240 W kg and is able to light up light-emitting diodes. Furthermore, the comparative electrochemical analysis of dynamic responses in different electrodes is helpful to further understand their charge-discharge mechanism and provide basic guidance to fabricate efficient electrodes for EESDs.