Directed Preparation Of Carbon@LDHs Core-shell Nanorod Arrays For Hihg Performance Supercapacitors

Supercapacitors(SCs)are considered to be the most promising energy storage and conversion devices because of their advantages such as high power density,fast charging and discharging speed,long cycle life,and wide operating temperature range.However,the low energy density is the main disadvantage,which is seriously restricted for the further application.The electrode material is the key to determine the performance of the SCs.Therefore,the optimization of electrode materials to improve the performance of SCs has attracted the most attention of researchers.Transition metal oxide/hydroxide and carbon materials are the main electrode materials.The transition metal oxide/hydroxide has a high therotical specific capacitance,but suffers from poor conductivity and poor cycle stability;while the carbon material has good conductivity and cycle stability but limites by lower specific capacitance.Combination of these two materials can exert their advantages and make up for disadvantages.However,how to obtain the hybrid electrode materials with excellent structure and performance through the materials design and preparation is still a big challenge.In response to the above problems,this thesis achieves the controlled preparation of carbon film-coated nanorod arrays and nano-branched carbon nanotube-coated nanorod arrays by useing one-dimensional nanorod arrays as templates.Furthermore,layered double hydroxides(LDHs)have been successfully synthesized on the surface of carbons,which gives uniform carbon@LDHs core-shell nanorod arrays.The hierarchical core-shell material exhibited an obvious synergetic effect,which combined the great electrical conductivity and stability of carbon electrode with the high specific capacitance and energy density of LDHs.By adjusting the factors such as morphology,graphitization degree of the carbon material,as well as coating amount of LDHs,the supercapacitor performance of the prepared composite material was further optimized.This thesis provided feasible and effective strategies for the preparation of well-ordered and excellent carbon/transition metal hydroxides composite electrodes for much enhanced SCs.The specific research content is as follows:(1)Directed synthesis of carbon@LDHs core-shell nanorod arrays for SCs.Firstly,the ZnO nanorod arrays were grown on foam nickel by hydrothermal method,and then ZIF-8 was coated on the ZnO by sacrificial template method.Then,ZnO@ZIF-8 was carbonized under high-temperature calcination under a nitrogen atmosphere,and the porous carbon nanorod arrays were obtained.The material properties were explored by changing the calcining temperature.It was found that when calcinated at 650 ℃,the graphitization degree of the carbon material is highest with a perfect structural stability.The satisfied graphitization degree facilitates the transport of electrons,which improved the electrical conductivity of the supercapacitor.The ZnO@C@LDH core-shell composites were further obtained by using the electrosynthesis of LDHs on the surface of ZnO@C.The ordered hierarchical structure made the as-prepared ZnO@C@LDH electrode a large specific surface area,which facilitates sufficient contact with the electrolyte solution.In addition,ZnO@ZIF-8 was also immersed in FeSO4 solution for Fe-Zn cation exchange.The resulting product were calcined under a nitrogen atmosphere at 450 0C to obtain Fe2O3@C core-shell nanorod arrays,which were used as the negative electrode to assemble a flexible solid-state device with the positive electrode material.The prepared flexible device had an excellent electrochemical energy storage performance.(2)Directed synthesis of CNTs@LDHs core-shell arrays for SCs.nano-branched cabon nanotube(CNT)arrays is successfully fabricated via simple heating volatilization deposition approach by using Ni foam supported Co3O4 nanowire array to catalyze growth of CNT and using waste gases that release from ZIF-67 precursor during pyrolysis as carbon sources.The as-obtained CNT nanoarrays was denoted as Co@CNTs,which showed a hierarchical branch-like morphology,consisting of well-distributed CNTs vertically grafted on the surface of Co3O4-derived nanowire array.Subsequently,the effect of calcination temperature and time for CNTs was explored,and the CNTs were finely regulated in length and density.Electrosynthetic method was used to effectively combine Co@CNTs with LDHs to obtain Co@CNTs@LDHs core-shell nanorod arrays.Due to the large specific surface area and excellent conductivity of the nano-branch CNT arrays,a larger loading of LDHs can be achieved than the previous work(from 2.8 mg cm-2 to 17.05 mg cm-2),which gave the composite material a very excellent electrochemical performance(specific capacity enhanced from 6.2578 F cm-2 to 23.09 F cm-2 at 2 mA cm-2).The carbon material obtained by high-temperature calcination of ZIF-67 as a negative electrode was assembled with a Co@CNTs@LDH core-shell nanorod arrays to form a flexible solid state device,which showed excellent supercapacitor performance.