The Preparation Of Nanometer Nickel (Cobalt) Hydroxide Electrode Materials And Their Electrchemical Properties

Transition metal oxide or transition metal hydroxide can cause highly reversible redoxreactions in the electrode/solution interface, the resulting Faraday pseudo-capacitance issignificantly higher than the electric double layer capacitance from carbon based materials.Therefore, the transition metal oxide electrode materials have attracted more attentions due totheir high capacitance. However, noble metal oxides, such as RuO2and IrO₂, are not suitablefor wide popularization and application due to their expensive cost. In this thesis, nickel（cobalt） hydroxide and graphene sheet/nickel （cobalt） hydroxide composite with differentmorphology as the electrode materials for supercapacitors were prepared through differentapproaches. Microstructure and morphology were characterized through the X-raydiffraction（XRD）, scanning electron microscope（SEM）, transmission electron microscope（TEM）, raman and X-Ray photoelectron spectrometer（XPS）. Electrochemical properties werecharacterized by cyclic voltammetry, constant current charge/discharge and electrochemicalimpedance spectroscopy. The detail research content and results are summarized as follows:Firstly, a simple hydrothermal synthesis method is adopted to prepare rod-thorn shapeCo（OH）₂and graphene/Co（OH）₂composite. The results show that conductive property ofCo（OH）₂is significantly improved after the introduction of graphene sheets. At scan rate of2mV·s^-1, the specific capacitiance of graphene/Co（OH）₂composite is increased from261F·g^-1to370F·g^-1. However, too much GNS will cause dispersion problems for this composite.SEM observations clearly show that the microstructure of graphene/Co（OH）₂is not uniform,and exhibits some large particles. The CV tests also show that when the content of GNS isincreased to14.6%, the capacity can only kept286F·g^-1at scan rate of2mV·s^-1.Secondly, Ni（OH）₂and GNS/Ni（OH）₂composite materials have been prepared throughhydrothermal method, coprecipitation method and microwave assisted method, respectively.Firstly, Ni（OH）₂from hydrothermal synthesis has an amorphous lamellar structure. Thesamples using different hydrothermal temperature and time have different electrochemicalproperties. The results indicate that the specific capacitance showed a downward trend withthe increse of reaction time and temperature. With an increase of reaction time andtemperature, large grain sizes lead to low specific capacitance. CV tests show that Ni（OH）₂prepared at100℃for4h exhibits good electrical properties. At the scan rate of2mV·s^-1, its capacitance can achieved1494F·g^-1. Secondly, for Ni（OH）₂prepared by coprecipitationmethod, we primary study the different concentration for the influence of microstructure.Ni（OH）₂presents a three-dimensional （3D） structure. After introduce of graphene sheets, thediameter of particles decreases and distributes uniformly on the sheets, suggesting thatgraphene sheets have effected on grain nucleation and growth stage. CV tests show that whenthe concentration is0.1M, the prepared Ni（OH）₂particles on the GNS are small andwell-distributed, its specific capacitance is reach up to1744F·g^-1at scan rate of5mV·s^-1.Thirdly, Ni（OH）₂from microwave synthesis shows flower-like microstructure, the diameter offlower is about300⁴00nm and thickness within7nm. After the introduction of graphenesheets, the diameter is reduced to200³00nm and thickness is reduced to3.5nm. At scanrate of2mV·s^-1, the specific capacitance increase from1583F·g^-1to1734F·g^-1.At last, we also have successfully assembled an asymmetric supercapacitor usingGNS/Ni（OH）₂as positive electrode and porous graphene as negative electrode, respectively.The asymmetric supercapacitor was investigated in6M KOH electrolyte between thepotential window of0¹.6V. Its specific capacitance can achieve218.4F.g^-1at scan rate of2mV·s^-1. The electrochemical analysis shows that asymmetric supercapacitor exhibits goodreversibility, stability and capacitance performance. Additionally, it presents a high energydensity of77.8Wh·kg^-1, which is significantly higher than those of the previously reportedNi（OH）₂based asymmetric supercapacitors. When the power density is15.2kW·kg^-1, energydensity can still maintain13.5Wh·kg^-1. Furthermore, this asymmetric supercapacitor exhibitsexcellent long cycle life along with93.6%specific capacitance retained after2000cyclestests.