Preparation And Electrochemical Performance Of Ni（OH）₂/Three-dimensional Carbon-based Composite Materials

Supercapacitors have been considered as the most promising energy-storage devices because of their high power density, better cycling performance than batteries and conventional capacitors. The key to improve the specific capacitance and cycle stability of supercapacitors is the preparation and optimization of electrode materials. Therefore, the objective of this thesis is to develop novel composite materials with superior electrochemical performance.First, the Ni(OH)2 with different morphologies were synthesized using a hydrothermal method. Their electrochemical performance tests showed that Ni(0H)2 nanowires possessed best cycle stability. Then, Ni(OH)2 nanowires were combined to both three-dimensional graphene and three-dimensional graphite foam to form composites using a hydrothermal method. The samples were characterized using X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, Raman and N2 physisorption measurements. Their electrochemical performance was tested by cyclic voltammetry and galvanostatic charge-discharge methods. The results are as follows:(1) The specific capacitance (908 F·g-1) and cycle stability (specific capacitance retention was 85% after 3000 cycles) of Ni(OH)2 nanowires were higher than that of other morphologies.(2) The Ni(OH)2 nanowires of width 20-30 nm were combined closely with graphene and crosslinked to one another to form a three-dimensional structure with a high specific surface area(136 m2·g-1) and mesoporosity (Pore diameter 20～50nm). The maximum specific capacitance of Ni(OH)2 nanowires/three-dimensional graphene composite was 1664 F·g-1 at 1 A·g-1. Moreover, the specific capacitance decreased by only 7% after 3000 cycles at 1 A·g-1.(3) Ni(OH)2 nanowires/graphite foam electrode exhibited specific capacitance up to 2144 F·g-1 at 1 A·g-1 owing to the high surface area (226.2 m2·g-1) and the three-dimensional framework, interconnected porous network (20～50 nm). An asymmetric supercapacitor based on Ni(OH)2 nanowires/graphite foam and activated carbon exhibited high cell voltage (1.8 V), energy density (59 Wh·kg-1) and power density (120 kW·kg-1). Furthermore, it also exhibited an excellent long cycle life along with 70% specific capacitance retained after 8000 cycles, which make it a promising electrode material for supercapacitors.(4) The composites with graphite like material and Ni(OH)2 nanowires have a larger specific surface area relative to Ni(OH)2 nanowires and three-dimensional mesoporosity, enabling an improved use of active material and significantly enhanced the electrochemical performance of Ni(OH)2 nanowires. The simple and general method can provide reference for the preparation of high performance composite electrode materials.