Synthesis of Ni(OH)2/RGO pseudocomposite on nickel foam for supercapacitors with superior performance

Abstract

A unique structure consisting of two kinds of Ni(OH)₂ layers on the top and the bottom, respectively, of the same reduced graphene oxide (RGO) layer has been designed and synthesized through a facile hydrothermal process. The lower layer of Ni(OH)₂, covered with a thin RGO film, is transformed in situ from the surface of a Ni foam substrate through the redox reaction of elemental Ni and graphene oxide (GO), while the upper layer of Ni(OH)₂ nanoflakes from Ni ions in the solution is vertically assembled on the top surface of the RGO of the lower RGO/Ni(OH)₂ layer. This composite can be regarded as combining RGO with a “pseudocomposite” of Ni(OH)₂ material because the upper and lower Ni(OH)₂ layers are different in morphology, particle size, and Ni²⁺ source. The bottom layer mainly acts as a rough support, while the upper Ni(OH)₂ is suitable to act as the main active material for supercapacitor electrodes. The lower layer of Ni(OH)₂/RGO, however, plays key roles in forming the aligned structure and in the subsequent cycling stability. The composite film has a high areal mass loading of 4.7 mg cm⁻², and superior supercapacitor performance. It features a specific capacitance of up to 15.65 F cm⁻² (i.e., 3328.7 F g⁻¹) at a current density of 7 mA cm⁻² (1.5 A g⁻¹) and a capacity retention of 90.6%, even after 5000 cycles at the high rate of 20 mA cm⁻² (4.3 A g⁻¹), indicating that it has a promising application as an efficient electrode for high-performance supercapacitors.