Fabrication And Performance Study Of Supercapacitors Based On Transition Metal Compounds And Carbon Materials

Nowdays,with increasingly exhausted fossil energy and unprecedented development of renewable energy system,higher requirements for energy storage devices have been put forward in all fields of society.In addition to civilian fields such as solar energy,tidal energy and hybrid electric vehicles fields,energy storage devices with high energy density,fast charging/discharging and ultra-long service life are also required in military fields such as heavy trucks and laser detectors.However,the widely used secondary batteries and traditional capacitors are difficult to simultaneously meet the urgent needs of the society.Therefore,supercapacitors with high power density and excellent cycle stability arise at the historic moment.Although supercapacitors have been applied in the fields of new energy vehicles,portable electronic products and intelligent power systems,the relatively low energy density restricts their wide commercial applications.The energy density of supercapacitors is proportional to their specific capacity and potential window.The key to increase the energy density is to improve charge storage capability of electrode active materials and expand the stable potential window of the devices.On this basis,the main research objectives of the dissertation are to construct nickel/cobalt-based transition metal compounds positive electrodes and three-dimensional porous carbons-based negative electrodes with excellent electrochemical performance,as well as to assemble supercapacitor devices with outstanding energy storage performance.The positive/negative electrode materials with high specific capacity as well as long operational life are prepared by synergic nanostructures design and surface functionalization strategies,so as to assemble high energy density symmetric/asymmetric supercapacitor devices.The specific research contents are summarized briefly as follows:(1)In order to improve the cyclic stability of transition metal phosphides,the asymmetric supercapacitors are constructed by using transition metal phosphides as positive electrode and biomass-derived porous carbon as negative electrode.In the mixed organic solvents of olamine and 1-octadecene,the Ni₂P/foam nickel(Ni₂P/NF)positive electrode is synthesized by a novel one-step colloid synthesis route using commercial nickel foam and tri-phenylphosphine as the reaction precursors.The obtained electrode delivers a satisfactory specific capacity of 1032 C g^-1(2293 F g^-1)at a current density of1 A g^-1 with excellent cyclic stability over 10,000 cycles.Besides,we also design a novel fabrication of sulfur doped porous actived carbon(HPAC)through pre-carbonization of concentrated sulfuric acid and high temperature pyrolysis.The constructed Ni₂P/NF//HPAC-800 asymmetric supercapacitors(ASCs)output an energy density of42.2 Wh kg^-1 as well as display an outstanding cycling performance.(2)Zeolitic imidazolate frameworks(ZIFs)have good structural flexibility,and it provides a new template for design of electrode materials with high specific surface area and rich Faradaic active centers.This work presents a systematic investigation of preparation and characterizations of single ZIF-67-derived CuCo₂S₄ nanoflower and N,F co-doped hierarchical porous carbon polyhedron(NFHPC).The CuCo₂S₄ nanoflower is prepared using the ZIF-67 as a sacrificial template through low temperature reflux and solvothermal reactions.Remarkably,the CuCo₂S₄ positive electrode exhibits a high specific capacity(672 C g^-1 at 1 A g^-1)as well as excellent cycling performance(93.1%of the initial capacitance after 5000 cycles).On the other hand,NFHPC is prepared by high temperature calcination and hydrothermal reaction using ZIF-67 as the template.The NFHPC negative electrode exhibits a high specific capacity of 305 F g^-1 at 1 A g^-1,and the rate performance remains 76.7%at 20 A g^-1.Moreover,the assembled ASCs output a high energy density of 55.1 Wh kg^-1 at 799.8 W kg^-1,as well as a long cycle life(remaining 91.1%after 5000 cycles).(3)Transition metal oxides and transition metal phosphides/sulfides have similar electrochemical energy storage behaviors.Transition metal oxides are promising electrode material for supercapacitors.Unfortunately,the intrinsic poor conductivity and deficient electrochemical active sites hinder the improvement of electrochemical performance.Spinel iron cobaltite(FeCo₂O₄)is a promising candidate for construction high-performance supercapacitors.Herein,an effective metal organic framework gel-derived strategy for preparation of FeCo₂O₄ surface functionalized with phosphate anions and oxygen vacancies(P-FeCo₂O_4-x)is reported.In the process of introduction of phosphate ions on P-FeCo₂O_4-x surface greatly enhances the surface activity,which prompts faster charge storage kinetics of electrode materials.Moreover,the formation of hierarchical porous structure enables the P-FeCo₂O_4-x structure to be retained during the ion intercalation.Benefiting from the above merits,the optimized P-FeCo₂O_4-x electrode delivers a high specific capacity of 784.4 C g^-1(1568.8 F g^-1)at 1 A g^-1 and remarkable cyclic stability(93.3%capacity retention after 5000 cycles).More importantly,the constructed P-FeCo₂O_4-x//AC ASCs achieve a favorable energy density of 60.2 Wh kg^-1at 800 W kg^-1,as well as a long cycle life span.(4)Porous carbon electrodes are widely applied in commercial electrochemical capacitors.Nevertheless,based on the research of HPAC and NFHPC,porous carbon materials are suffered with complex synthesis methods and laborious removal of activated templates.Therefore,the synthesis of heteroatom doped carbon materials from cheap biomass by appropriate activation method is important for the sustainable development of carbon-based supercapacitors.Herein,this work presents an environment-friendly and scalable technical to fabricate N/O co-doped hierarchical porous three-dimensional carbon nanofoam(ZCH-PC)using[Zn CO₃]₂·[Zn(OH)₂]₃ as the activation template and biomass as the carbon source.The ZCH-PC electrode exhibits a high specific capacity of367 F g^-1 at 0.5 A g^-1,and only decays 3.8%of initial specific capacity after 10000 cycles.Notably,the assembled ZCH-PC//ZCH-PC symmetric supercapacitors achieve a high energy density of 24.6 Wh kg^-1,and 95.7%of initial capacitance is maintained after 10000cycles.