Electrochemical Charge Storage Behavior of Various ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$ 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.

Figure 1

(a)-(f), (i) and (j) SEM images, and (g), (h), (k) and (l) TEM images: (a) spherelike ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$, (b) hatlike ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$, (c) and (d) PAN NFs, (e)-(h) brushlike NiCo₂S₄ and (i)-(l) flowerlike ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$.

Figure 2

(a)–(e) CV curves at different scan rates, and (f) the values of b obtained from the CV curves for the four ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$ electrodes.

Figure 3

(a)–(d) The values of k₁ obtained from CV curves at different potentials, and (e) and (f) the percentages of capacitive contribution at different scan rates for the four ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$ electrodes.

Figure 4

GCD curves at different current densities for the four ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$ electrodes.

Figure 5

(a) Specific capacitance/capacity values derived from the GCD curves at different current densities for the four ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$ electrodes, respectively. (b) Cycling stability and Coulombic efficiency within 10 000 cycles at 10 A g⁻¹ for the brush- and flowerlike, respectively.

Figure 6

(a) GCD curves at a current density of 1 A g⁻¹, and (b) voltage drop (IR drop) as a function of discharge current for the four ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$ electrodes.

Figure 7

Nyquist plots (inset is the corresponding equivalent circuit) for different ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$ electrodes.

Figure 8

(a) Schematic illustration and (b)–(d) electrochemical performance of the asymmetric supercapacitor based on the brushlike ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$ electrode: (b) GCD curves at different current densities, (c) CV curves at different scan rates, and (d) cycling stability at 10 A g⁻¹ (inset is the picture of the supercapacitors lighting a LED). (e) GCD and (f) CV curves of two asymmetric supercapacitors in series.

Figure 9

Schematic diagrams of the ion diffusion and charge transport in the electrochemical process for different ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$ electrodes.