Preparation Of Cobalt-based Electrode Materials And Their Supercapacitor Performance Study

The important position of energy storage in today's human society has motivated the research of sustainable and renewable power sources to the forefront.As an alternative to batteries and traditional electrostatic capacitors,supercapacitors have attracted tremendous attention because of their good pulse charge/discharge characteristics,long life cycle?millions of cycles?,and high power capabilities.These merits are essential for practical applications,rendering supercapacitors the most promising backups or auxiliary power sources for electric vehicles and electronic devices.However,even such power and energy levels of supercapacitors are still insufficient to meet emerging needs.To achieve high-performance energy-harvesting,a well-defined and tailored nanostructure is the prerequisite for electrode.Therefore,this research focuses on the smart modification of traditional capacitor systems and electrode materials,including hybridization of bespoke pseudocapacitive nanoarchitectures,optimizing the structure of graphene-based materials,and fulfilling all-solid-state asymmetric supercapacitors with high flexibility.The core work of this paper is mainly focus on the following parts:?1?Anovel flower-on-sheet hierarchical morphology of?-Co?OH?₂ nanostructures was achieved via an easy two-step synthesis strategy.The method is based on first a galvanostatic electrodeposition?GE?of vertically aligned interconnected Co?OH?₂ nanosheets to form a branch layer and second a potentiostatic electrodeposition?PE?of Co?OH?₂ microflowers on the obtained branch layer from the secondary growth of their sheet-like precursors.The formation mechanism of this special PE time-dependent nanostructure was proposed and their morphology-dependent supercapacitor properties were also investigated.For a given areas mass loading,high specific capacitances of 1822 F/g have been achieved for the electrode obtained after 200 s GE followed by a 300 s PE in a three-electrode configuration,and it maintained 91%of its initial capacity after 1000 constant-current charge/discharge cycles.?2?The template free electrodeposition path has been accepted for the formation of hierarchical Co?OH?₂ flower-on-sheet configuration on flexible carbon textiles.Such special Co?OH?₂ architecture can deliver a high specific capacitance of 1482 F/g in the solid-state two-electrode system.Meanwhile,the hierarchical Co?OH?₂-based electrode was assembled with a new-type highly porous graphene paper into an flexible asymmetric supercapacitor,which demonstrates high specific energies of 220 Wh/kg at 411 W/kg and 84 Wh/kg at 3.37kW/kg and outstanding cycling performance.?3?Solid-state flexible energy storage devices hold the key to achieve portable and flexible electronic capacitors.A highly ordered mesoporous NiCo₂O₄ nanoarchitecture anchored on carbon fabric has been developed successfully by a template-free method,showing a distinctive hierarchical morphology with numerous wing-like secondary grown nanosheets that shortens electrolyte ion diffusion paths and facilitates their rapid migration.Besides,a novel free-standing graphene paper is obtained by a post freeze-drying treatment that endows it with a porous interconnected network,ensuring both surfaces of graphene sheets be readily accessible by the electrolyte and providing a larger surface area for charge-transfer reactions.An asymmetric supercapacitor composed of graphene paper cathode and NiCo₂O₄/CC anode with PVA-LiOH gel served as both electrolyte and separator was also studied,in order to fulfill the synergistic effects between the two components to improve the rate capability and cycling stability of the device.?4?Here a high-density aligned Co-Al layered double oxide sheets and NiCo₂O₄ flakes core/shell nm-scale arrays,and a novel freeze-dried graphene/multi-walled carbon nanotube porous sponge,are introduced historically the first time.Benefiting from vertically aligned two flake-like nanocrystals with sufficient free spaces in the hierarchical pseudocapacitance and cross-linked monolithic graphene in the 3D carbon-carbon network,the assembled flexible symmetric cells yield colossal capacitance behaviors and good rate capability.By complementary tailoring of the asymmetric electrodes,the solid-state device delivers superior cycling performance,high energy and power densities,suggesting that such hybrid system possesses a great potential application.?5?A novel hollow polyaniline?PANI?nano-capsule with holes on the wall has been synthesized via a facile interfacial polymerization?IP?method without using any templates.The intriguing microstructure of such nano-capsule has been investigated by scanning electron microscopy?SEM?and transmission electron microscopy?TEM?.A possible growth mechanism of the PANI nano-capsule has also been proposed.In supercapacitro electrochemical test,the obtained PANI nano-capsule electrode exhibits a high specific capacitance of 502 F/g(at the current density of 5 mA/cm² and excellent cycling stability,rendering it to be a promising material for supercapacitor applications.