Study On Preparation Of NiCo₂S₄-based Composites And Their Supercapacitive Performance

Supercapacitor is considered to be a promising energy storage device due to its fast charge and discharge,excellent cycle stability,high power density,and green friendliness.Electrode material is the core part of supercapacitor.At present,metal compounds exhibit higher specific capacitance than the counterparts due to their rich redox reaction,and are the most promising electrode active materials for supercapacitors.In this thesis,nickel-cobalt-based sulfides were used as the electroactive materials for supercapacitors,and their composites were also discussed.The morphology and electrochemical properties of the composites were systematically investigated.The major findings are as follows:（1）Eggplant was used as the carbon source and carbon flakes with ultra-high specific surface area were obtained by KOH activation method.The carbon flakes were used as substrate to synthesize EC@NiCo₂S₄ composite by two-step hydrothermal method.SEM inspection showed that the size of NiCo₂S₄ is smaller when confined by carbon flakes than that grown without the carbon flakes,and was arranged in a nanosheet array on the eggplant carbon flakes.From the results of electrochemical performance,the composite electrode had better specific capacitance and cycle stability than the pure NiCo₂S₄ electrode.At 1 A g^-1,the specific capacitance of the composite reached 1394.5 F g^-1.The composite material was used as the positive electrode,and the eggplant carbon was used as the negative electrode to assemble the asymmetric supercapacitor.The device had an energy density of 46.5 Wh kg^-1 at a power density of 801 W kg^-1.（2）Dopamine was used as the carbon source and nitrogen-doped hollow carbon nanospheres（NHCSs）were synthesized by silica hard template method.The NHCSs were used as the substrate to synthesize NiCo₂S₄/NHCS,which was achived by anchoring of the nickel-cobalt precursor on the surface of NHCSs followed by anion exchange with Na₂S solution.Pure NiCo₂S₄ was agglomerated with no regular morphology when no carbon substrate was used,whereas small-sized,ultra-thin NiCo₂S₄ nanosheets coated on the NHCSs when the substrate was used.It was indicated that the specific capacitance of NiCo₂S₄/NHCS was 901.5 F g^-1 at a current density of 1 A g^-1,which was significantly higher than that of pure NiCo₂S₄(656.2 F g^-1).Moreover,NiCo₂S₄/NHCS had superior rate performance than pure NiCo₂S₄.NiCo₂S₄/NHCS was used as the positive electrode,and activated carbon was used as the negative electrode to assemble asymmetric supercapacitor.The device had good energy density and cycle stability.（3）Using carbon cloth as the substrate,a layered NiCo₂S₄@NiMoO₄ composite material was synthesized by hydrothermal reaction as a binder-free electrode.SEM tests showed that NiMoO₄ nanosheets evenly covered the surface of NiCo₂S₄ nanotubes,increasing the specific surface area of composites,which was beneficial to electrolyte ion transport and increase specific capacitance.It had a specific capacitance of 1419 F g^-1 at 2 A g^-1.The assembled asymmetric supercapacitor had an energy density of 29.6 Wh kg^-1 at a power density of 750.9W·kg^-1,and a good specific capacitance(94.6 F g^-1 at 1 A g^-1)and cycle stability（Capacitance retention was 82.2%after 5,000 cycles）.