Preparation And Electrochemical Properties Of The Flexible Nickel Cobalt Sulfide/Carbon Nanofiber Electrodes

The ever-growing demand for sustainable energy has pushed forward the development of efficient energy-storage devices.Supercapacitors（SCs）with fast charge and discharge rate capability,which have great potential in energy storage.Recently,flexible and wearable electronic devices have become a noticeable technological trend that has experienced rapid growth.Hence,flexible supercapacitors have been an important role in wearable devices.The capacitance performance,flexibility,and stability of flexible supercapacitors can be affected by flexible electrodes as one of the key components.In this thesis,nanofiber materials decorated by nickel cobalt sulfide have been obtained by electrostatic spinning technology,carbonization,and hydrothermal treatment.Because of their excellent flexibility,the nanofiber materials decorated by nickel cobalt sulfide can be used as the flexible electrode and their chemical composition and electrochemical performances have been discussed as well.Ternary nickel cobalt sulfides enjoy superiority in comparison with their oxide counterparts due to better electronic conductivity and higher theoretical specific capacitance.The seasame-ball-like core-shell hollow mesoporous carbon spheres decorated by NiCo₂S₄ nanoparticles（NiCo₂S₄/HMCSs）were prepared via a hydrothermal method.The obtained NiCo₂S₄/HMCSs could be used as the positive electrodes for asymmetric supercapacitors（ASCs）,combining the outstanding electroconductivity of carbonic materials and the high capacity of NiCo₂S₄nanoparticles.The electrochemical properties of the as-obtained composites showed an outstanding capacity of 1098.5 F·g^-1(0.5 A·g^-1).The calculated capacitance of HMCSs was 194.5 F·g^-1(0.5 A·g^-1).Finally,ASCs were further fabricated by using the NiCo₂S₄/HMCSs as the cathode and hollow mesoporous carbon spheres（HMCSs）as the anode.Moreover,the assembled ASCs exhibited a good energy density of 35.1Wh·kg^-1 at a moderate power density of about 203.5 W·kg^-1and good cycle ability with～90%even after 3000 cycles.The flexible electrospun carbon nanofiber substrates fixed with Ag nanoparticles and graphene oxide have been synthesized via a combination of electrospinning and carbonization treatments.And then the growth of NiCo₂S₄nanotube on carbon nanofibers to form hybrid carbon nanofibers were prepared by a hydrothermal method.The obtained hybrid carbon nanofibers exhibited excellent property of suppleness.The electrochemical activity and stability of hybrid carbon nanofibers were investigated and it exhibited an excellent specific capacity of 360 F·g^-1 at 0.3 A·g^-1.Notably,the hybrid carbon nanofibers exhibited excellent long-term cycling stability（90.0%retention after2000 cycles）.The assembled symmetric hybrid carbon nanofibers-based supercapacitor also exhibited excellent flexibility,include bendable,foldable,and even twistable property.The symmetrical supercapacitor operation voltage is low,not conducive to practical application,and storage capacity effect is not good.Asymmetric supercapacitors have higher energy densities and operating voltage.Based on this theory,the hybrid flexible carbon nanofibers decorated with NiCo₂S₄ nanoparticles are successfully prepared by a combination of electrospinning,carbonization,and hydrothermal treatments.The obtained hybrid carbon nanofibers display remarkable specific capacitance(527.8 F·g^-1 at the current density of 0.2 A·g^-1 and 622.5 F·g^-1at 2m V·s^-1),good rate capability and good cycle stability（retaining 90%after 3000 cycles）.The flexible asymmetric supercapacitors based on carbon nanofibers decorated with NiCo₂S₄ and porous carbon nanofibers coated with activated charcoal（AC@p CNFs）are fabricated.The AC@p CNFs possess capacitance of 144.5 F g^-1 at 0.48 A g^-1(160.5F g^-1at 2 m V s^-1).The fabricated device shows an energy density of 32.1 Wh·kg^-1 at a power density of 67.6 W·kg^-1 and cycle stability retaining 88.5%after 1500 cycles at1.0 A·g^-1.Therefore,the novel flexible asymmetric device with perfect flexibility and stability can be applied as one of the most promising power supplies.The preparation of flexible carbon fiber-based supercapacitor by industrial mature electroplating technology has obvious advantages in the preparation method.A novel flexible electrode material for supercapacitor is prepared by electrodeposition of Ni-Co-S interconnected nanosheet arrays on N-doped hierarchical porous carbon nanofibers（Ni-Co-S@N-p CNFs）.The electrode exhibits a remarkable specific capacitance of 520.2 F·g^-1 at the current density of 0.2 A·g^-1 and 670 F·g^-1 at 2m V·s^-1.The activated charcoal（AC）loaded N-doped porous carbon nanofibers（AC@N-p CNFs）as the electrode is also prepared which exhibits a specific capacitance of 257.2F·g^-1 at 0.2 A·g^-1,as well as excellent cycling stability with capacitance retention of92.6%after 5000 cycles.The combined unique properties of each of these components enable highly flexible and mechanically strong films that can serve as electrodes directly without using any current collectors or binders.Furthermore,the Ni-Co-S@N-p CNFs as the positive electrode,active carbon loaded N-doped hierarchical porous carbon nanofibers（AC@N-p CNFs）as negative electrode are assembled into the flexible asymmetric supercapacitor.The assembled supercapacitor device shows a high energy density of 21.6 Wh·kg^-1 at a power density of 134.9 W·kg^-1and robust long-term cycling stability retaining over 90.0%after 3000 cycles at 1.0 A·g^-1.The as-assembled asymmetric supercapacitors（ASCs）have outstanding mechanical flexibility and electrochemical stability and no significant attenuation occurred during cycling under different bending states.Ion substitution is one of the highly effective approaches in promoting the electrochemical performance of electrodes.Herein,we reported the preparation of phosphorus-substituted Ni-Co-S nanosheet arrays（Ni-Co-S-P）that electrodeposited on nitrogen-doped carbonic nanofibers（N-CNFs）for applications in supercapacitor and hydrogen evolution reaction.The as-synthesized Ni-Co-S-P nanosheet arrays demonstrated an over-potential of 165 m V at a current density of 10 m A·cm^-2 for water splitting half-reaction（HER:hydrogen evolution reaction）with long cycling stability over 20 h.Benefiting from the unique composition and structure,the as-obtained Ni-Co-S-P electrode displayed a high capacity performance of 1590 F·g^-1 at 0.5 A·g^-1.For considering the application in ASCs,the negative electrode materials were prepared by electrodepositing iron oxides nanosheets on N-CNFs（denoted as Fe-O/N-CNFs）.The as-prepared Fe-O/N-CNFs exhibit the capacity of 692.7 F·g^-1 at 0.5 A·g^-1.Thus,the Fe-O/N-CNFs possess the potential to serve as a negative electrode.A hybrid supercapacitor based on the obtained electrodes was further assembled.The as-fabricated hybrid supercapacitor showed a high energy density at 55.8 Wh·kg^-1and power density at 402.9 W·kg^-1,respectively.