Study On Preparation And Electrochemical Performance Of Layered Double Hydroxides

As an important device for power storage,supercapacitors have many advantages of high power density,long cycling life and fast charging ability.However,low energy density limits their practical application.The methods to improve the energy density of supercapacitors mainly include increasing the specific capacitance of electrode materials and fabricating the asymmetric hybrid supercapacitors.In asymmetric hybrid supercapacitor,activated carbon is used as electrode and the other electrode is from pseudocapacitive material with an expanding operating voltage,thereby increasing the energy density.In this thesis,porous carbon materials with superior electrochemical performance and layered double hydroxides(LDHs)with different morphologies were successfully prepared.Then they were assembled into asymmetric hybrid supercapacitors.Both aforementioned methods were simultaneously utilized to enhance the energy density of supercapacitors in this work.Through the characterization of electrode materials and the test of electrochemical properties,the following results were obtained:(1)Using egg white as raw material,porous carbon materials containing nitrogen(EWC)were synthesized by simple KOH activation method,which has high specific surface area and nitrogen content.Its specific capacitance at 0.5 A·g^-1 current density is 335 F·g^-1 and the assembled symmetrical supercapacitor exhibits a high energy density of 13.6 Wh·kg^-1.(2)Co Ni LDHs was synthesized by solvothermal method using 2-methylimidazole as functional additive,methanol as solvent and acetate as interlayer anion.Co Ni LDH-6,obtained by regulating the amount of 2-methylimidazole,has high specific capacitance,superior reversibility and cycling stability.The asymmetric hybrid supercapacitor assembled with Co Ni LDH-6 as positive electrode and EWC as negative electrode has a high energy density of 51.1 Wh·kg^-1 at a power density of 1.7 k W·kg^-1.(3)Composite materials(NMN)with core-shell structure were prepared by multiple solvothermal methods using cobalt-nickel sulfide as core and nickel-manganese layered double hydroxide as shell.NMN-2 exhibits better electrochemical performance by adjusting the content of additional NH₄F,which has a specific capacitance of 1261.5 F·g^-1at 1 A·g^-1.An assembled NMN-2//EWC asymmetric hybrid supercapacitor achieves maximum energy density of 44.5 Wh·kg^-1 at a power density of 850.2 W·kg^-1.(4)A composite consisting of cobalt-nickel layered double hydroxides(CoNiLDHs) and nitrogen-containing porous carbon materials(NMC)was obtained by using a simple coprecipitation method.The electrical conductivity of composite is improved and the severe aggregation of LDHs nanosheets is effectively prevented.Thus more active sites are exposed,which results in an improved electrochemical performance.The specific capacitance of the NMC-Co Ni LDH-2 composite is 1421.2 F·g^-1 at a current density of 1A·g^-1.The fabricated NMC-Co Ni LDH-2//NMC asymmetric hybrid supercapacitor exhibits high energy density of 51.38 Wh·kg^-1 at the power density of 1700 W·kg^-1.Morevoer,this hybrid device also achieves a decent cycling stability.