Construction And Energy Storage Mechanism Research Of Nickel-Cobalt Hierachical Structure Based Electrode Materials

Supercapacitors is a new class of materials that promises to be applied in the energy storage and conversion field due to their rapid charge-discharge rate,high power density,long lifespan and safety.The electrode material plays a key role in determining the energy storage capacity,and more and more advanced hierarchical nanostructured electrode materials have been studied.Nickel-cobalt based electrodes with the advantages of because of their low cost,nontoxicity and easy preparation methods of different morphologies have been taken as one kind of the most promising electrode materials,and more importantly,they show excellent electrochemical performances.Among these,nickel-cobalt phosphides and selenide have been intensively studied.The application of nickel-cobalt phosphides and selenide electrodes is limited by their relatively low electrical conductivity and structural stability.In this thesis,we designed and constructed hierarchical nanostructured nickel-cobalt hydroxides,phosphides and selenide with different morphologies from the perspectives of composition regulation and structural design to improve the electric conductivity and structural stability and the largely boosted supercapacitive performances were achieved and the the mechanism of energy storage of electrode material were revealed.（1）NiCoP@NiCoP core-shell array was rationally designed and successfully fabricated by a facile two-step hydrothermal method.The superior electrochemical performance of the NiCoP@NiCoP@CC electrode is attributed to the combined benefits of transition phosphides with high conductivity and an exquisite three-dimensional hierarchical architecture:the porous core-shell structure serve as“ion-buffering reservoirs”to store the electrolyte ions to shorten the ionic diffusion distance from the external electrolyte to the interior surfaces the core,and the outer ultrathin NiCoP nanosheets can improve the contact area with the electrolyte and promotes the penetration of the ions and the transport of charge.The as-fabricated electrode shows superior electrochemical performance and delivers a high specific capacity of 1125 C g^-1(312 mAh g^-1)at 1 A g^-1,and outstanding rate capability with78.0%retention even at 10 A g^-11 and still retain 808 C g^-1(224 mAh g^-1)（71.8%retention）after 2000 cycles.（2）NiCoP@C@LDHs electrode with hierarchical core-shell structure,which NiCoP@C nanowires array is taken for the core,to support the NiCo-based nanosheets shell has been successfully assembled,and the effects of the introduction of carbon-based materials on the electrical conductivity and mechanical stability has been explored.Amorphous carbon layer served as a buffer layer between the NiCoP nanowires and LDHs nanosheets,which protects the structure from collapsing during the charge-discharge process.（3）Explore the influence of transition metal ion doping on the morphology and electronic structure of prepared nanomaterials,and further explore the influence of the introduction of transition metal ion on the electrochemical performance of electrodes.Mo-doped LDHs electrode shows a high area capacity and good rate capability with92.0%retention at 20 mA cm^-2 and improved cycle stability.（4）We have managed to conceive and fabricate a core-shell structured triangle-like array,where selenylation-treated MOF array in situ grown on a conducting substrate as high conductive framework to support high loading Mo-doped nickel-cobalt LDHs nanoflakes,for the purpose of combining the superiorities of porous structure and structural advantages.The differences and advantages of morphology,structure and reaction mechanism using electrodeposition and hydrothermal method has also been developed.The electrode with high loading,high area capacity and structural stability has been fabricated.Mo-doped LDHs@MOF-Se electrode reveals a fairly high area capacity of 5.16 C cm^-2(10.32 F cm^-2)at 2 mA cm^-2,which displays approximately 91.9%capability retention at 10 mA cm^-2,besides,the electrode can keep up 81.4%of original capacity behind 3000 times of charge and discharge processes at 20 mA cm^-2.