Three-Dimensional Hierarchical NixCo1-xO/NiyCo2-yP@C Hybrids on Nickel Foam for Excellent Supercapacitors

Abstract

Active materials and special structures of the electrode have decisive influence on the electrochemical properties of supercapacitors. Herein, three-dimensional (3D) hierarchical Ni_xCo_1-xO/Ni_yCo_2-yP@C (denoted as NiCoOP@C) hybrids have been successfully prepared by a phosphorization treatment of hierarchical Ni_xCo_1-xO@C grown on nickel foam. The resulting NiCoOP@C hybrids exhibit an outstanding specific capacitance and cycle performance because they couple the merits of the superior cycling stability of Ni_xCo_1-xO, the high specific capacitance of Ni_yCo_2-yP, the mechanical stability of carbon layer, and the 3D hierarchical structure. The specific capacitance of 2638 F g^-1 can be obtained at the current density of 1 A g^-1, and even at the current density of 20 A g^-1, the NiCoOP@C electrode still possesses a specific capacitance of 1144 F g^-1. After 3000 cycles at 10 A g^-1, 84% of the initial specific capacitance is still remained. In addition, an asymmetric ultracapacitor (ASC) is assembled through using NiCoOP@C hybrids as anode and activated carbon as cathode. The as-prepared ASC obtains a maximum energy density of 39.4 Wh kg^-1 at a power density of 394 W kg^-1 and still holds 21 Wh kg^-1 at 7500 W kg^-1.

Keywords: carbon coverage; excellent performance; nickel cobalt oxide; nickel cobalt phosphide; supercapacitor.