Preparation Of Nickel Hydroxide/nanocarbon Material Composites And Study Of Their Capacitance Properties

Supercapacitor, being an energy storage device, has many advantages such as fast charge-discharge, high power density and environmental friendly, and so on. The supercapacitor is more and more important in energy storage with the developing modern society. Among such influening factors, electrode material is the most crucial one on supercapacitor performance. At present, the research of asymmetric supercapacitor positive materials, are mainly focused on the preparation of transition metal oxide/hydroxide-supported carbon materials. Ni(OH)2 has been widely investigated due to its high theoretical specific capacitance and low cost. But its electrical conductivity is not good and the Ni(OH)2 particles are easy to accumulate. Commercial activated carbon (AC) with large specific surface area and carbon black Vulcan XC-72 (XC-72) with good conductivity, will be chosen as support to load Ni(OH)2, respectively. The obtained Ni(OH)2/AC and Ni(OH)2/XC-72 composites are expected to get improved specific capacitances and stability.In this thesis, ethylene glycol (EG) was used as a reducing agent to synthesis Ni(OH)2/AC and Ni(OH)2/XC-72 composites via a microwave-assisted heating method. The preparation parameters, conductive agent types and their addition approaches were investigated. Various material analysis techniques like XRD, SEM, FT-IR, Raman and TGA were applied to characterize the morphologies and structures of the composites and electrodes. And electrochemical methods like cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) tests were employed to measure the electrochemical performances. Enventually, the composite with best electrochemical performances and commercial AC were respectively used as positive and negative electrode material to assemble an asymmetric supercapacitor, hoping to acquire a device with high energy density and capacitance. The detailed results are as follows:(1) AC as support to prepare Ni(OH)2/AC composite. The electrochemical results demonstrate that Ni(OH)2/AC composite exhibites the best electrochemical performances when the Ni(OH)2 mass ratio of 60 wt%, deposition time of 8 h, microwave heating time of 4 min and precipitation pH value of 11.3. When CNT was added as both conductive agent and support during deposition process, the obtained 60 wt% Ni(OH)2/AC/CNT electrode showes better electrochemical performances. It can achieve 1038.3 F/g at current density of 1 A/g and capacitance retention remains 64.7% after 1000 cycles.(2) Vulcan XC-72 as support to prepare Ni(OH)2/XC-72 composite. The optimal preparation parameters are that Ni(OH)2 mass ratio is 60 wt%, deposition time is 10.5 h, microwave heating time lasts 3 min and pH value of the precipitation medium is 11.3. When CNT as a conductive agent was added during prepare the electrode slurry, the obtained 60 wt% Ni(OH)2/XC-72 electrode embraces a 3D connected network structure and showes better electrochemical performances. The maximum specific capacitance can reach up to 1560 F/g at 1 A/g and the capacitance retention remains 71% after 1000 cycles.(3) Ni(OH)2/XC-72 composite and AC were respectively employed as positive and negative electrode material to assemble the AC//Ni(OH)2/XC-72 asymmetric supercapacitor. The device was tested at potential window of 1.6 V, m-/m+ mass ratio of 2.25,6 M KOH as an electrolyte and non-woven fabric as a separator in a two-electrode system. Its specific capacitance can achieve 92.2 F/g at 0.5 A/g and the maximum energy density can reach up to 36 Wh/kg. The results manifest adequately the good electrochemical performances of AC//Ni(OH)2/XC-72 asymmetric supercapacitor.In conclusion, Ni(OH)2/XC-72 composite with 3D connected network structure and CNT as conductive agent is turned out to have good electrochemical performances due to the unique electrode structure. Furthermore, the asymmetric supercapacitor using Ni(OH)2/XC-72 composite as positive electrode material also exhibites high energy density, which reveals that Ni(OH)2/XC-72 composite will have development prospect in supercapacitor applications.