Synthesis of Cobalt Sulfide and Cobalt Sulfide/Graphene Nanocomposite and Their Application for Supercapacitors

Author:Zhu Jia

Supervisor:yang juan

Database:Doctor

Degree Year:2019

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Pages:112

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As a potential energy storage device,supercapacitors have attracted many attentions of researchers due to the advantages on ultrafast charge-discharge ability,long lifespan,high power density.Electrode materials play an important part in supercapacitors,among which cobalt-based electrode materials are regarded as a promising candidate for supercapacitors owing to their high specific capacitance,relatively low cost and high reversible redox capability.Nevertheless,the practical applications of cobalt-based electrode materials are still hindered by the problems of tedious synthesis procedure,poor mechanical properties,various forms cobalt sulfide of product,which result in unsatisfactory specific capacitance,rate capability and cycling performance.In this dissertation,we design and prepare cobalt-based electrode materials with satisfactory performance for supercapacitors,including cobalt sulfides and cobalt sulfide/reduced graphene composites.We try to find high efficient and facile ways to obtain cobalt-based electrode materials.Also,we optimize the synthetic procedure to realize the controllable synthesis of cobalt sulfides and cobalt sulfide/reduced graphene composites with the aid of amine compound,investigate the synthetic mechanism,and study the effects of physical characteristics of the electrode materials on their electrochemical performances.The main contributions are described as follows:(1)Micrometer sized flower-like CoS hierarchitectures have been successfully synthesized through a hydrothermal route at the presence of ethylenediamine as a structure-directing agent.The electrochemical testing results suggest that CoS hierarchitectures show typical pseudocapacitive features and the specific capacitance reaches 414 F·g-11 at 0.5 A·g-1.After being cycled for 1000 cycles at 0.5 A·g-1,the specific capacitance remains 81.4%of its initial value.(2)CoS cluster has been successfully synthesized via a simple one-step hydrothermal route assisted by diethylenetriamine(DETA)as a structure-directing agent.Under SEM observation,CoS cluster typically exhibit coarse and uneven surface and severely agglomerate microstructure.The results of the electrochemical measurements reveal that the specific capacitance of CoS cluster reaches as high as664 F·g-11 at 0.5 A·g-1,and even at a high current density of 10 A·g-11 CoS cluster can offord 471 F.g-1,displaying perfect rata capability.Besides,CoS-modified electrode shows satisfactory cycling durability with about 86%specific capacitance retention of its initial value after 1000 cycles.(3)Co3S4 materials have been successfully produced via a facile one-step hydrothermalapproachassistedbytriethylenediamine(TEDA)asa structure-directing agent.The effects of synthetic conditions like molar ratio of raw materials,reaction time,and reaction temperature on the structure and purity of the products have been investigated.Also,the optimal synthesis method was obtained.The structural characterizations demonstrate that Co3S4 materials present cubic crystal form,polyhedron shape,and a certain degree of agglomeration.In contrast to the counterpart of Co3S4,Co3S4 materials have better single-phase crystallographic property,higher purtiry and more regular morphology.The electrochemical tests indicate that the special Co3S4-based electrodes deliver typical pseudocapacitance features with a high specific capacitance of 1038 F·g-11 at 0.5 A·g-1.Additionally,it exhibits remarkable cycling stability with 89.8%specific capacitance retention of its initial value after 1000 cycles,showing perfect comprehensive performance.(4)A series of cobalt sulfide decorated reduced graphene oxide(CoS/rGO)nanocomposites have been successfully synthesized via a facile one-step hydrothermal route with the aid of ethylenediamine by adjusting the mass ratio(r)of Co(CH3COO)2.4H2O to GO.Physical characterizations disclose that CoS/rGO nanocomposite exhibits a restacked microstructure formed by two dimensional graphene sheets.CoS nanoplates interlace and connect each other to constitute the CoS nanoparticles,which have a broad size distribution(30100 nm).Due to the geometric confinement of rGO,the size and agglomeration of CoS particles have been greatly decreased.CoS nanoparticles are uniformly embedded in,wrapped in or loaded on wrinkled and restacked graphene sheets.CoS/rGO nanocomposite with r=6owns the maximal specific capacitance.CoS/rGO-based supercapacitorharvests a maximum specific capacitance of 813 F·g-11 at 0.5 A·g-1(about 2 times of pure CoS)and91.2%capacitanceretentionofitsinitialvalueaftercontinuous charging/discharging 1000 cycles,displaying superior specific capacitance,cycling performance and rate capability over contrastive pristine CoS(the specific capacitance of 414 F·g-11 at 0.5 A·g-1,and the specific capacitance remaining 81.4%of its initial value after being cycled for 1000 cycles at 0.5 A·g-1).Electrochemical impedance spectroscopy was applied to analyze the reasons for the enhanced electrochemical performance of CoS/rGO nanocomposites.The results cogently indicate that CoS/rGO-based electrode compared to CoS imparts smaller solution resistance and interfacial charge-transfer resistance,thus showing more ideal charge-transfer kinetics,faster ions transport rates,higher specific capacitance and better cycling performance.