Preparation And Electrochemical Properties Study Of Cobalt Sulfide/Carbon Composite Nanofibers

The development of high performance composite electrode materials is critical for improving the performance of electrochemical energy storage and conversion devices.In the previous literature,cobalt sulfide is considered as a promising faradaic pseudocapacitance supercapacitor electrode material and a high active HER(hydrogen evolution reaction)catalyst.Therefore,developing low-cost synthesis method and rational designing the microstructure of cobalt sulfide nanomaterials can further optimize its electrochemical performance.Based on the well-established electrospinning process,this thesis developed two different synthesis routes(two-step spinning-sulfuration method and one-step direct sulfuration method)based on two kinds of electrospinning precursors.The obtained cobalt sulfide(Co9S8)nanoparticles/carbon composite nanofibers are employed as supercapacitor electrode and electrochemical HER catalyst,respectively.The composite structure not only utilizes the high conductivity and good chemical stability of the one-dimensional carbon nanofibrs,but also combines the high electrochemical activity characteristics of the Co9S8 nanoparticle.The electrochemical activity and the cycle stability were improved by constructing such composite structure.The main research contents in this thesis are as follows:(1)Co3O4/carbon precursor nanofibers were prepared by electrospinning and the subsequent stabilization heat treatment.In this step,conventional polyacrylonitrile was used as the carbon source while cobalt acetate was used as the cobalt source.In the second step,Co9S8/carbon composite nanofibers were prepared by the mild hydrothermal sulfuration using thioacetamide as the sulfur source,and following the subsequent carbonization treatment.By optimizing the reaction parameters,the loading of cobalt sulfide can be readily controlled.Such composite nanofibers showed high capacitance and high rate capability in supercapacitor electrode.For instance,the composite nanofibers have a specific capacitance of 718 F g-1 at a current density of 1 A g-1 and a capacitance retention of 40.7%when the current density reached to 20 A g-1.An asymmetric supercapacitor device was assembled with activated carbon nanofibers as the negative electrode,and it can operate at 1.5 V voltage window and have an energy density of 23.8 Wh kg-1 at a power density of 0.75 kW kg-1.(2)Sodium lignin sulfonate,a by-product from the paper industry,was used as the dual functional reagent for both carbon and sulfur sources in this thesis.Cobalt nitrate/lignin sulfonate composite precursor nanofibers were prepared by electrospinning without additional sulfur source.After that,Co9S8/carbon composite nanofibers were directly obtained by a one-step carbonization method and used as HER electrocatalyst.A good HER activity was achieved by adjusting the content of the active material in the catalyst.The optimized overpotential was 187 mV at the current density of 10 mA cm-2,and the Tafel slope was 114 mV dec-1.