Ethylene glycol assisted self-template conversion approach to synthesize hollow NiS microspheres for a high performance all-solid-state supercapacitor

Abstract

Traditional hard/soft template approaches used for the synthesis of hollow structures suffer from tedious procedures and are time-consuming. Compared to these methods, self-template approaches usually have the advantage of step-economy. Herein, solid spherical Ni-glycolate/Ni microspheres were initially synthesized by reacting nickel(II) with ethylene glycol at high temperature. The source of nickel(II) has a significant impact on the formation of Ni-glycolate/Ni. In particular, nickel acetate as nickle source yielded Ni-glycolate/Ni, while nickel nitrate could not. The plausible reason for this can be attributed to the difference in acidity of the system caused by the dissolution of nickel acetate and nickel nitrate. By reacting with thioacetamide (TAA) under solvothermal conditions and heat treatment, the Ni-glycolate/Ni microspheres were further employed as a self-template to direct the formation of hollow NiS microspheres. The driving force for converting Ni-glycolate/Ni into hollow NiS could be attributed to the Kirkendall effect resulted from the different diffusion rates of nickel ions (from Ni-glycolate/Ni) and sulfur ions from the decomposition of TAA. When applied as an electrode material in supercapacitors, the formed hollow NiS microspheres exhibited a specific capacitance of 1674 F g⁻¹ at 1 A g⁻¹ with excellent rate capability and cycling performance. The corresponding asymmetric device of NiS//AC delivered a high energy density of 43.3 W h kg⁻¹ at the power density of 850.3 W kg⁻¹. These achievements pave the way for an innovative synthetic approach to hollow structures for many other practical applications.