| Supercapacitors(SCs),also called electrochemical capacitors,have received intense attention because of their higher energy density than conventional dielectric capacitors and higher power density than secondary batteries.These superior advantages enable them promising in various areas,such as back-up power supplies,hybrid electric vehicles,and portable electronic devices.As is known,the morphology and structure of electroactive materials have a key influence on electrochemical behaviors.Owing to the low electronegativity of sulfur,transition metal sulfides exhibit a more flexible structure and much higher conductivity compared with corresponding oxides.Among them,Ni sulphides and Co sulphides have aroused great interests due to their high theoretic specific capacitance value,abundant raw materials,environmental friendly and safety,which are expected to satisfy the increasing needs of energy storage systems.In this paper,NiS2/COS2 composites with different Ni and Co molar ratios for supercapacitors(SCs)were synthesized by one-step hydrothermal co-deposition method.With the increase of Ni content,the composites particle size increases gradually and the hollow sphere structure becomes more obvious.Meanwhile,the capacitance performance of SCs also can be controlled by adjusting the ratios of Ni and Co.Secondly,we synthesized NiS microflowers through a facile sacrificial template,using Ni(OH)2 as the precursor.The SEM analysis results demonstrated that the NiS microflowers were composed of hierarchical nanoplates,and its surface was much rougher compared to that of precursor after sulfidation.Finally,the difference of surface areas,surface roughness and capacitance performance of NiS microflowers can be fabricated by adjusting the vulcanization time(the time were 12,18 and 24 h,respectively).Impressively,with the reaction time extending to 18 h,the NiS-18 microflower is of the largest surface areas and highest specific capacitance.The main results are as follows:(1)Synthesis of NiS2/COS2 composites and their application in supercapacitors.NiS2/COS2 composites with different Ni and Co molar ratios for supercapacitors(SCs)were synthesized by one-step hydrothermal co-deposition method.With the increase of Ni content,the composites particle size increases gradually and the hollow sphere structure becomes more obvious.The electrochemical measurements demonstrate that these composites possess a high specific capacitance(Cm)performance,long cycle stability and good rate capability.To be specific,the Cm of Ni/Co/S-1 composite is the largest,up to 954.3 F·g-1 at 1 A·g-1,and as high as 309.5 F·g-1 even at large current density of 20 A·g-1.Furthermore,the Ni/Co/S-1 maintains 99.9%of its initial C,after 1000 cycles at 5 A·g-1.Moreover,the asymmetric supercapacitors with Ni/Co/S-1 as positive electrode and actived carbon as negative electrode are of prominent energy density of 29.3 Wh·kg-1 at the power density of 0.7 kW·kg-1,and superior cycling stability of 99.1%initial value retention after 1000 cycles.(2)Synthesis of hierarchical NiS microflowers for high performance supercapacitor.The hierarchical NiS microflowers were synthesized using Ni(OH)2 as precursors through sulfuration process.The SEM analysis results demonstrated that the NiS microflowers were composed of hierarchical nanoplates,and its surface was much rougher compared to that of Ni(OH)2 templates after sulfidation.Electrochemical characterization revealed that NiS presented high specific capacitance in 3 M KOH electrolyte(1122.7 F·g-1 at current density of 1 A·g-1)and good electrochemical cycling stability(97.8%of the specific capacitance is retained after 1000 charge-discharge cycles at current density of 10 A·g-1).Furthermore,an asymmetric supercapacitor,NiS as the positive electrode and activated carbon as the negative electrode,delivered high energy density of 31 Wh·kg-1 at power density of 0.9 kW·kg-1 under operating voltage window of 1.8 V.(3)Hierarchical NiS microflowers:tuning the vulcanizing time for high performance supercapacitor.The hierarchical NiS,Ni(OH)2 microflowers as precursors,were synthesized under different vulcanizing time through a facial sacrificial template method.The SEM results show that the NiS microstructure changes from microflower to microspheres gradually with the increase of vulcanizing time.Impressively,with the reaction time extending to 18 h,the NiS-18 microflowers is of the largest surface areas(20.5 m2·g-1)and the roughest surface.Electrochemical measurements showed that NiS-18 possessed the highest specific capacitance(1315.4 F·g-1 at current density of 1 A·g-1)and excellent cycling stability(89.2%of the Cm is retained after 5000 GCD cycles at current density of 10 A·g-1).More importantly,an asymmetric supercapacitor,assembled by NiS-18 as positive electrode and active carbon(AC)as negative electrode(NiS//AC).The NiS//AC displays a high energy density of 33.4 Wh·kg-1 at power density of 0.8 kW·kg-1.Additionally,NiS//AC exhibits excellent cycling performance(87.3%Cm retained even after 5000 GCD cycles at current density of 5 A·g-1)and nearly 100%Coulombic efficiency. |
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