Figure 4: Electrochemical characterizations of the NiCo₂S₄ ball-in-ball hollow spheres electrode.

(a) Cyclic voltammetry curves. (b) Galvanostatic charge/discharge voltage profiles. (c) Specific capacitance as a function of current density. (d) Cycling performance at a current density of 5 A g⁻¹. SCE, saturated calomel electrode