Synthesis and electrochemical properties of Ni-Co layered double hydroxides with high performance

Supercapacitor is a new-type energy storage device with the promising application prospect, and its development mainly relies on the development of electrode materials. In this work, a series of nickel-cobalt (Ni-Co) layered double hydroxides is synthesized via a simple hydrothermal method by using nickel and cobalt salts with four different anions (including sulfate, chlorate, acetate and nitrate) serving as nickel and cobalt sources. According to the types of salts, the obtained samples are named Ni-Co(SO4), Ni-Co(Cl), Ni-Co(Ac) and Ni-Co(NO3), respectively. The morphology and structure of Ni-Co layered double hydroxide are characterized by X-ray diffraction and scanning electron microscopy (SEM), respectively, and the electrochemical properties of the sample are investigated by CHI660D electrochemical workstation in 2 M KOH aqueous solution. The results demonstrate that the types of nickel and cobalt salts not only affect the morphology and structure of Ni-Co layered double hydroxide, but also significantly influence the electrochemical properties of the sample. The SEM images show that the Ni-Co layered double hydroxide synthesized with nickel sulfate and cobalt sulfate (Ni-Co(SO4)) possesses loose layer structure, which can provide abundant active sites and benefit the diffusion of electrolyte. The electrochemical test results show that the specific capacitances of Ni-Co(SO4), Ni-Co(Cl), Ni-Co(Ac) and Ni-Co(NO3) under a current density of 1 A/g at a potential window of 0.45 V, are 1551.1 F/g, 440.7 F/g, 337.8 F/g and 141.6 F/g respectively. As the current density increases from 1 A/g to 7 A/g, the capacitive retention rates of Ni-Co(SO4), Ni-Co(Cl), Ni-Co(Ac) and Ni-Co(NO3) are kept at 60.1%, 21.7%, 4.6% and 6.0%, respectively. The results of alternating current (AC) impedance test display that the electron transfer resistance follows an increasing trend:R[Ni-Co(SO4)] R[Ni-Co(Cl)] R[Ni-Co(Ac)] R[Ni-Co(NO3)]. The small electron transfer resistance is conducive to excellent capacitance at the high current density. Therefore, the excellent capacitive performance of the sample Ni-Co(SO4) is ascribed to the loose layer structure and low electron transfer resistance. In addition, the cycling stabilities of the samples are investigated by constant current charge-discharge test. The capacitive value of the sample Ni-Co(SO4) declines by 16% for 1000 cycles at a current density of 7 A/g. The capacitance decrease can be ascribed to the damage to the layered structure and the increase of the electron transfer resistance in the multiple constant current charge-discharge processes as shown in the results of SEM and AC impedance before and after cycle. This study provides a foundation for exploiting and utilizing high-performance nickel-cobalt layered double hydroxides as electrode material of supercapacitor.