The Preparation And Capacitance Performance Study Of Threedimensional Cobalt-based Nanomaterials

Supercapacitor with high power density,fast charging-discharging and long cycle life has become one of the most promising energy-storage devices.The electrode materials play a vital role in supercapacitor;among which cobalt-based material is considered to be a capacitor electrode material with excellent potential in view of its high specific capacitance,relatively modest cost,environment friendly trait and safety.Nevertheless,the practical application of cobalt-based materials has been limited owing to some problems such as particle agglomeration,single component instablity and preparation process,which makes the experimental specific capacitance unsatisfactory.The purpose of this dissertation is to prepare high-performance cobalt-based capacitor materials.By reasonable designing cobalt-based electrode materials,3D structures of cobalt bisulfite/graphene（CoS₂/RGO）,cobalt trisulfide/graphene（Co₃S₄/RGO）and the tertiary micro-nano structure Ni foam/manganese cobalt oxide/cobalt sulfide（Ni/Mn Co₂O₄/CoS） materials have been synthesized with high specific surface area,ordered/porous structure and high chemical stability.Then the synergistic effect between different components was explored.Finally,asymmetric supercapacitor device was manufactured and tested.Cobalt disulfide（CoS₂）and CoS₂/RGO have been prepared by directly growing on the Ni foam current collector in a facile hydrothermal approach,which avoids the use of binders and conductive agents and simplifies the process.The electrochemical testing results demonstrate that the CoS₂/RGO composite material obtains the capacitance of 930.3 F·g^-1(2 A·g^-1).Physical characterization indicated that the CoS₂nanoparticles in the composite electrode are evenly coated by graphene sheets to form a novel large-scale wave-like structure.Owing to the geometric confinement of CoS₂ and RGO,the size and agglomeration of CoS₂ in the composite electrode are significantly reduced to obtain higher specific surface area between the composite electrode and the electrolyte,which is beneficial for forming 3D conductive network.In addition,asymmetric supercapacitor has been designed and assembled using cobalt bisulfite/graphene composite electrode-2（CoS₂/RGO-2） and activated carbon to explore the practicality of CoS₂/RGO-2.Electrochemical testing results display that the asymmetric supercapacitor material delivers an energy density of 45.7 Wh·kg^-1at the corresponding power density of 797.0 W·kg^-1and 90%retention of the initial available capacitance after 6000 charge-discharge tests at 10 A·g^-1,showing excellent overall performance.A in situ hydrothermal growth process is adoped to obtain a 3D porous coral-like trisulfide（Co₃S₄）,which directly grows on the Ni faom current collector.SEM displays that the growth process is“nucleation-crystallization–crystal growing and nanosheet formation–nanaosheets-nanosheets crossing–porous coralline structure”and the morphology of Co₃S₄ has been influenced by temperature,solvent and p H.The electrochemical measurements demonstrate that the 3D porous coral-like Co₃S₄electrode material exhibits a higher specific capacitance of 1513.6 F·g^-1at 0.5 A·g^-1,as well as excellent rate capability and remarkable cycle performance.The as-fabricated asymmetric supercapacitor device based on Co₃S₄ and activated carbon displays the maximum energy density is 59.2 Wh·kg^-1at the corresponding power density of 418.2 W·kg^-1,as well as remarkable rate capability and cyclic stability.On this basis,Co₃S₄/RGO composite electrodes are also prepared.The physical characterization shows that the composite electrodes have not completely retained the 3D porous coral-like structure.While,most of the Co₃S₄/RGO composite electrodes exist in the form of dispersed nanosheets.Meanwhile,the specific capacitance of the composite electrode is not much improved.Mn Co₂O₄ has been prepared by in situ hydrothermal growth on the Ni foam skeleton surface.By controlling the concentration of NH₄F and changing the degree of complexation with the metal ions,Mn Co₂O₄ nanowire and Mn Co₂O₄ nanosheet are obtained,respectively.Then,two tertiary micro-nano structure Ni/Mn Co₂O₄/CoS composite electrode:Ni/Mn Co₂O₄ nanowire/CoS and Ni/Mn Co₂O₄ nanosheet/CoS composite electrodes have been engineered and prepared by electrodeposited CoS on the Mn Co₂O₄ nanowire and Mn Co₂O₄ nanosheet,respectively.The electrochemical tests display that the Ni/MnCo₂O₄ nanosheet/CoS composite material exhibits highest specific capacitance of 1607.4 F·g^-1at current density of 0.8 A·g^-1,excellent rate capability and cycling stability compared with the Mn Co₂O₄ nanowire,Ni/MnCo₂O₄nanowire/CoS,Mn Co₂O₄ nanosheet electrodes.Further,the solid-state asymmetric supercapacitor is fabricated using Ni/Mn Co₂O₄ nanosheet/CoS and activated carbon as positive and negative electrodes,respectively.The materal achieves remarkable energy density of 55.1 Wh·kg^-1at power density of 477.3 W·kg^-1.It is worth pointing out that the loss of the initial capacitance is only 9.0% after 6000 cycles at a current density of 8 A·g^-1,indicating that the device exhibits remarkable electrochemical stability.Electrochemical impedance spectroscopy and galvanostatic charge-discharge measurement were used to analyze the reasons for the improvement of the performance of the tertiary micro-nano structure Ni/Mn Co₂O₄ nanosheet/CoS composite material.The result indicates that the composite electrode has smaller ohmic resistance and electrochemical transfer resistance,which are beneficial for ion/electron transport and charge transfer.The DFT calculations show that the energy band structure and density of states at the interface of the composite change significantly,at the same time,it is found that there are more electrons near the Fermi level,indicating better electronic conductivity and higher electrochemical activity.