Facile hydrothermal synthesis of NiMoO4@CoMoO4 hierarchical nanospheres for supercapacitor applications.

A novel binder-free electrode material of NiMoO4@CoMoO4 hierarchical nanospheres anchored on nickel foam with excellent electrochemical performance has been synthesized via a facile hydrothermal strategy. Microstructures and morphologies of samples are characterized by X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Besides, the effect of Ni/Co molar ratios of raw materials on electrochemical behaviors is also investigated by cyclic voltammetry, galvanostatic charge-discharge measurements, cycling tests and electrochemical impedance spectroscopy methods. Remarkably, the resulting NiMoO4@CoMoO4 hierarchical nanospheres with a Ni/Co molar ratio of 4 : 1 exhibit greatly enhanced capacitive properties relative to other components and display a high specific capacitance of 1601.6 F g(-1) at the current density of 2 A g(-1), as well as better cycling stability and rate capability. Moreover, a symmetric supercapacitor is constructed using NiMoO4@CoMoO4 nanospheres as the positive and negative electrodes with one piece of cellulose paper as the separator, which shows good electrochemical performance. Such enchanced capacitive properties are mostly attributed to the synergistic effect of nickel and cobalt molybdates directly deposited on the conductive substrate and their novel hierarchical structure, which can provide pathways for fast diffusion and transportation of ions and electrons and a large number of active sites. The results imply that the NiMoO4@CoMoO4 hierarchical nanospheres could be promising candidates for electrochemical energy storage.