Constructions And Supercapacitor Properties Of NiCo₂S₄ Nanocomposite Electrode For Application

Although many transition metal sulfides have been investigated as electrodes for supercapacitors(SCs),binary metal sulfides are quite interesting,due to their higher active redox sites,as well as mechanical and thermal stability compared to that of their corresponding single component counterparts.Nickel-cobalt sulfide(NiCo₂S₄,NCS)is the most promising electrode material,which has excellent electrochemical characteristics,such as a small bandgap(1.2 e V),and excellent electrical conductivity(1.23×10~6Sm^-1),easy to synthesize and multiple oxidation states,such as nickel(Ni²⁺/Ni³⁺)cobalt(Co²⁺/Co³⁺/Co⁴⁺),which make them one of the most promising electrode material for high performance supercapacitor.However,NCS also has the drawback such as fast conductivity decay,material instability,and poor cycle stability during electrochemical activities.In order to overcome these problems,it is necessary to combined with some conductive additives such as carbon materials due to their high surface area,electrical conductivity,good stability and construct new nanostructures which significantly improve the performance of NCS based electrodes.Herein,we constructed different nanostructures including NCS/carbon nanotubes(CNT)nanocomposite structure,and NCS based SiO₂@C core-shell structure,so as to systematically study its supercapacitor characteristics,and finally to improve the drawback of NCS and improved their performance from the current existing reported literature.The main work of the thesis is divided into three parts:1)Tuning the NCS fabrication by controlling the pricess parameter.Study investigated the morphology dependent electrochemical behavior tuned with different stirring times(1 min,5 min,10 min,20 min,40 min,and 1 hr.)NCS hexagonal plate and NiCo-OH nanoparticles(NiCo-OH NPs).The altered nanostructure properties are well supported by in-depth structural and morphological analysis.When used as electrodes for supercapacitor,both NCS hexagonal plates and NiCo-OH NPs(prepared at stirring time of 20 min and 5 min,respectively)exhibit maximum specific capacitances of 833 and 733 m F cm^-2at the current density of 10 m A cm^-2along with superior rate and stable cycling performance due to their beneficial structural features.Moreover,the stability study showed 69.0%and 67.0%capacitance retention after 5000cycles,indicating excellent stability of the samples.These outstanding properties manifest that the unique hierarchical structures promised as a suitable electrode material for the energy storage field shortage.2)Constructed NCS/CNTs nanocomposite structures by utilizing the high conductivity of CNTs.NCS/CNT nanocomposite electrodes with different CNT contents were obtained by hydrothermal synthesis.The structure and morphology characterization showed that NCS was deposited on CNTs to platelets like morphology.When used as an electrode material for supercapacitors,the NCS/CNT-1 composite(CNT content of 1%)exhibits a high specific capacitance value of 1690 F g^-1at a current density of 5 A g^-1.More importantly,an asymmetric supercapacitor(ASC)assembled based on with NCS/CNT-1 as the positive electrode and carbon nanotube paper(CNP)as the negative electrode exhibits a high energy density of 58 Wh kg^-1energy density with power density of 8 k W kg^-1and good cycle stability(capacitance retention rate was 77.7%of the initial value)after 7000 charge-discharge cycles under a current density of 8 A g^-1.The large enhancement in the electrochemical performance is attributed to the benefits of the nanostructured architecture,including good mechanical stability,high electrical conductivity as well as buffering for the volume changes during charge–discharge process.These convincing results show that NCS/CNTs hybrid nanostructures are promising electrode materials for high energy density SCs.3)Constructed a new nanocomposite(SiO₂@C-NCS)as electrode material based on SiO₂@C core-shell nanospheres.Benefiting from compositional and structural advantages,the as-prepared SiO₂@C-NCS nanocomposite exhibits high specific capacitance of 625 F g^-1at a current density of 1 m A cm^-2,with long-term stable cycling performance.When using SiO₂@C-NCS as the positive electrode CNP as the negative electrode to assemble an ASC,it shows a high energy density of 16 Wh kg^-1in a wide voltage range of 1.6 V and high power density of 7200 W kg^-1.The research in this thesis finds that for NCS,SiO₂@C-NCS and NiCo-OH NPs electrode materials,the design of nanostructures can increase the specific surface area of??the materials,and at the same time effectively reflect the synergistic properties of nano and micro structures,thereby preventing the electrode materials from agglomerating,improving energy/power densities,and cycling stability.It is worth noting that the existence of porous structure is beneficial to excellent cycle stability and high specific capacitance.In addition,the porous structure not only maintains the supply of electrolyte ions and provides sufficient far-point reaction for energy storage,but also acts as an ion buffer reservoir,providing more diffusion channels and reducing the diffusion distance of ions.This allows sufficient electrolyte ions to make contact with the inner/outer surface of the electrode material.Finally,this paper provides a reference for other(transition metal chalcogenides)TMCs to improve the overall electrochemical performance including high energy/power density,excellent cycling stability,high conductivity and excellent capacitance retention ability.