The Preparation, Surface Modification And Supercapacitance Properties Of Cyclodextrin-based Carbon Nanospheres

Supercapacitor is a new energy storage device with the advantages of short charging and discharging time,high power density and long service life.It has great application prospects in electronic equipment,electric vehicle and aviation industry,as well as national defense and military.However,low energy density hinders its development in practical applications.According to the energy density formula?E=1/2CV²?,the key to increase the energy density is to increase the specific capacitance and broaden the voltage window.This dissertation aims to increase the capacitance and operation voltage window.Specifically,the purpose of this work is to prepare carbon-based symmetric supercapacitors and metal oxide-based asymmetric supercapacitors with high power density and energy density.The electrochemical performance of the electrode material was optimized by structural design,morphology control and heteroatom doping,so as to enhance the specific capacitance of the material.In addition,appropriate electrolytes and asymmetric supercapacitors were selected to broaden the voltage window of the device.The details are as follows:1.Three kinds of carbon spheres were prepared from F127 and cyclodextrin by hydrothermal process and heat treatment?including KOH activation?.Carbon spheres are three-dimensional structures,which endow them with excellent stability in the process of electrochemical reaction.Secondly,the activation of KOH provides the carbon sphere with large specific surface area and developed porosity,which is conducive to full contact between electrolyte and electrode material and rapid diffusion of electrolyte into electrode material.Oxygen doping not only increases the pseudocapacitance behavior,but also inhibits the hydrogen evolution or oxygen evolution reaction and enlarges the voltage window of carbon-based symmetric supercapacitors.For the reasons mentioned above,PCNS?Soft-template?exhibits excellent electrochemical performance in 6 M KOH.It not only exhibits a capacitance of 405 F g^-1 at 1A g^-1,but also exhibits excellent rate performance at a super high current density of 200 A g^-1.The symmetrical supercapacitors based of PCNS?Soft-template?electrode exhibit a voltage window of 1.2 V in 6 M KOH with ultra-high power density of 301500 W kg^-1,while in 1M Li₂SO₄,the voltage window is widened to 1.6 V with high energy density of 24.27 Wh kg^-1.2.PCNS was modified with Co-doped NiMoO₄ nanosheets by hydrothermal process,which showed hollow structure.PCNS@Co_0.21Ni_0.79MoO₄ exhibits an ultra-high specific capacitance of 954 F g^-1 at 1A g^-1 when 21%of Ni in NiMoO₄ is replaced by Co.It has excellent rate capability and excellent cycle stability?101%capacitance retention after 5000 cycles?,which is twice that of the undoped PCNS@NiMoO₄.Asymmetric supercapacitor?ASC?devices with PCNS@Co_0.21Ni_0.79MoO₄ as positive electrode and commercial activated carbon?AC?as negative electrode have excellent energy density and cyclic stability.Cobalt doped nickel molybdate hollow nanospheres were hydrothermally grown using porous carbon spheres as the substrate.Carbon spheres can not only improve the conductivity of nickel molybdate,but also form core-shell structure with nickel molybdate,provide high specific surface area and more electroactive sites,allow electrodes to contact electrolytes sufficiently,and make full use of the synergistic effect of various components to improve the electrochemical performance;while the nickel molybdate hollow nanospheres have larger internal and external specific surface area and could effectively contact with electrolytes,buffering volume change of materials in the electrochemical process,which improves the cyclic stability.Four-point probe test and density functional theory?DFT?calculation show that cobalt doping improves the intrinsic conductivity of nickel molybdate,and endows it with excellent rate performance and cyclic stability.