Fabrication Of Ni（OH）₂-Based Composite Material Through Hydrothermal Method With Complex Agent And Their Asymmetric Supercapacitor

With the rapid development of economy,the energy resources are gradually exhaused,and the humen beings are increasing demand for environment,there is an urgent need for green renewable energy and energy storage device to address the problems.Supercapacitors,possessing fast charge-discharge rate,long cycle stability,higher power density and environmentally friendly,have been applied in many fields such as stand-by power systems,portable electronics,medical treatments and military devices.Supercapacitors are composed of electrode material,current collector,membrane,electrolyte and shell,the electrochemical performance of supercapacitors mainly depends on the electrode material.The nickel-based electrode materials have attracted researcher’s attentions due to abundant raw materials,low toxicity and higher theoretical specific capacity.In this thesis,the Ni（OH）₂ and Ni（OH）₂-based composites have been synthesized via hydrothermal method with complex agent and exhibited excellent electrochemical performance.The asymmetrical supercapacitor was also assmbled by the prepared Ni-based materials as positive electrode and active carbon as negative electrode,and the electrochemical properties of the asymmetrical supercapacitor was explored.（1）The porous nanosheets ofβ-Ni（OH）₂ was prepared via hydrothermal method at70 ^oC for 2 h in Ni Cl₂·6H₂O-NH₃·H₂O-C₂H₅OH system,and the temperature and time condition of hydrothermal method was explored.The hydrothermal temperature and time significantly affect the crystallinity ofβ-Ni（OH）₂ and has slight impact the morphology ofβ-Ni（OH）₂.Cyclic voltammetry（CV）,galvanostatic charge-discharge tests（GCD）and electrochemical impedance（EIS）ofβ-Ni（OH）₂ prepared at different reaction temperature and time condition were investigated.The prepared sample for maintaining at 70 ^oC for 2 h shows the excellent electrochemical performance with specific capacity of 763 C·g^-1 at 1 A·g^-1and 575 C·g^-1 at 10 A·g^-1.In addition,the asymmetrical supercapacitor was also fabricated usingβ-Ni（OH）₂ as the positive electrode and activated carbon（AC）as negative electrode,which delivers the high power density of 7983 W·kg^-1 and energy density of 34.8 Wh·kg^-1.（2）In the above system for preparingβ-Ni（OH）₂,Co Cl₂·6H₂O was added,three kinds of cobalt nickel bimetallic hydroxides（Ni_xCo_y-LDHs,x:y=0.89:0.11,0.80:0.20,0.67:0.33）were prepared by adjusting the molar ratio of Co and Ni.The morphology and structure of the samples were analyzed by XRD,SEM and XPS.CV,GCD and EIS of the samples were investigated.The kinds of Ni_xCo_y-LDHs are similar in morphology,but the electrochemical performances have remarkable difference.In the three electrodes system,the electrochemical properties of Ni_0.80Co_0.20-LDHs shows good specific capacity of 820 C·g^-1 at 1 A·g^-1 and 554 C·g^-1 at 20 A·g^-1,which is good rate capability of 67.6%.Moreover,asymmetrical supercapacitor assembled by Ni_0.80Co_0.20-LDHs as the positive electrode and AC as negative electrode,delivers the high power density of 8012 W·kg^-1 and energy density of 35.9 Wh·kg^-1.（3）In the above system for preparingβ-Ni（OH）₂,g-C₃N₄ was added,the porous nanosheets of g-C₃N₄/β-Ni（OH）₂ composites were fabricated via hydrothermal method at 140 ^oC for 6 h,and final content of g-C₃N₄ in g-C₃N₄/β-Ni（OH）₂ composites was determined by thermogravimetric analysis.The morphology and structure of the samples were analyzed by XRD,SEM,TEM,FT-IR and XPS.The g-C₃N₄（5.3wt.%）/β-Ni（OH）₂ composite exhibits the excellent electrochemical performance.the specific capacitance of g-C₃N₄（5.3wt.%）/β-Ni（OH）₂ composite is 341C·g^-1 at 1 A·g^-1,which is 12 times and 1.8 times than that of pure g-C₃N₄(28 C·g^-1)andβ-Ni（OH）₂(212 C·g^-1),respectively.Moreover,the asymmetrical supercapacitor based on g-C₃N₄（5.3wt.%）/β-Ni（OH）₂ as the positive electrode and AC as negative electrode was constructed,which exhibits the high power density of 7992 W·kg^-1 and energy density of 28.3 Wh·kg^-1.The prepared g-C₃N₄（5.3wt.%）/β-Ni（OH）₂ composite presents excellent electrochemical performance due to porous nanosheets,which not only provide channel for the fast transport and diffusion of electrolyte ions but also improve the contact area between electrolyte ions and electrodes,and enhance Ni（II）/Ni（III）reaction taken.