Nickel-cobalt-sulfide Micro/Nano Materials Derived From MOFs And Their Applications In Electrochemical Energy Storage

As an energy storage device with a good application prospect,supercapacitors have attracted the attention of researchers.At present,the exploration of excellent electrode materials is a hot topic in the research field of supercapacitors.As materials for electrode,metal sulfides often have higher theoretical capacity than oxides,but their rate performance and cycle stability still needs to be improved.Since the composition and morphology can affect the properties of the materials,it has attracted a wide range of research interests to synthesis the composition and morphology controllable sulfide nano-materials.In this thesis,a precursor sulfuration route was used to synthesize the metal sulfide micro-nanomaterials with the controllable composition and morphology.And the performance of the obtained metal sulfide micro-nanomaterials as an electrode for energy storage device was studied.In that way,metal sulfide electrode materials with superior electrochemical performance wre obtained.The specific content is summarized as follows:1.Hollow Ni/Co-S microspheres prepared through a precursor sulfuration route were reported.The precursor was first generated at 150?C for 24 h by a solvothermal process,employing Ni?NO₃?₂,Co?NO₃?₂ and 1,3,5-Benzenetricarboxylic acid?BTC?as the original reactants,and ethylene glycol?EG?as the solvent.Then,the precursor was sulfurized in a water-EG mixed solvent at 180?C for 18 h to form hollow Ni/Co-S microspheres.It was found that the electrochemical properties of hollow Ni/Co-S microspheres strongly depended on the original molar ratio of Ni²⁺/Co²⁺.The product prepared from the system with the Ni²⁺/Co²⁺molar ratio of 2:1?labeled as 2Ni-Co-S?exhibited the best electrochemical performances.At a current density of 1 A g^-1,the specific capacitance of 2Ni-Co-S reached 2794 F g^-1;and even at a current density of 10 A g^-1,the specific capacitance still reached 1816 F g^-1.After 1500 cycles,the specific capacitance still retained¹50%of the initial value and⁸8.0%of the biggest capacitance,exhibiting excellent cycling stability.Also,an asymmetrical supercapacitor assembled with 2Ni-Co-S and activated carbon owned the energy density of 146.6 Wh kg^-1 at a power density of 1.6 kW kg^-1;and even at a power density of 8 kW kg^-1,the energy density still achieved 128 Wh kg^-1,indicating that the as-obtained hollow Ni/Co-S microspheres have potential applications for energy storage as high performance electrode materials.2.Star-like Co-Ni-S nanoparticles were successfully synthesized through a two-step solvothermal route.Some factors influencing the formation of star-like Co-Ni-S nanoparticles were systematically investigated.First,flake-like Co/Ni-PTA precursor was solvothermally synthesized at 150?C for 24 h,employing Ni?NO₃?₂,Co?NO₃?₂ and terephthalic acid as the original reactants,and ethylene glycol?EG?as the solvent.Then,as-obtained Co/Ni-PTA precursor was sulfurized in a water-EG mixed solvent at 180?C for various durations to form Co-Ni-S nanoparticles.It was found that the sulfuration time could strongly affect the component and electrochemical performances of the final product under keeping the Ni²⁺/Co²⁺molar ratio of 2:1 unchanged.The product prepared by sulfurizing for 18 h?marked as S-18h?presented the highest specific capacitance and owned good cycle stability.In addition,an asymmetrical supercapacitor assembled with S-18h and activated carbon exhibited the energy density of 42.94 Wh kg^-1 at a power density of 400 W kg^-1;and even at a power density of 4 kW kg^-1,the energy density still reached 23.28 Wh kg^-1.The above fact shows that the obtained S-18h star-like nanoparticle is a high-performance electrode material for asymmetrical supercapacitors.3.NiS/NiO@CNT composite nanomaterials with good electrochemical performances were designed and synthesized:First,the Ni-BTC precursor was carbonized at 800?C in a tube furnace to gain NiO/Ni@CNT composite intermediate;then,NiO/Ni@CNT was partially sulfurized with sulfur powder to obtain NiS/NiO@CNT.As a control,NiO/Ni@C composites were also obtained by carbonizing the Ni-BTC precursor at 400?C.The electrochemical test showed that the initial specific capacitance of the NiS/NiO@CNT composite of 1303.0 F g^-1 could be gained at the current density of1 A g^-1,which was higher than 863.6 F g^-1 of the NiS/NiO@C.The improved electrochemical performances should be attributed to the presence of carbon nanotubes.In addition,an asymmetrical supercapacitor assembled with NiS/NiO@CNT composites and activated carbon?AC?exhibited a capacitance of 93.38 F g^-1 at the current density of 1 A g^-1.Also,NiS/NiO@CNT composites exhibited good rate performance and good cycle stability.After 2000 cycle,the specific capacitance still remained 88.8%of the maximum value.The above fact indicates that the present Ni S/NiO@CNT composites have potential application as an electrode material for supercapacitors.