Preparation And Characterization Of Flexible Fiber-shaped Supercapacitors Based On Transition Metal Compounds

The undue consumption of fossil energy by human activities resulted in serious environmental pollution,which compelled people to develop green and sustainable new energy sources and efficient energy storage devices as soon as possible.Supercapacitor as one of promising energy storage devices has unique features compared with others,such as better cycle life and greater power density than battery,and higher energy density than traditional capacitor.Furthermore,it also exhibits better stability,wider operating temperature range and superior safety.Supercapacitor has attracted more and more attention from global researchers and becomes one important field of new energy research.In particular,the rapid progress in integrated circuits and related scientific technological fields has accelerated the research and development of portable wearable electronic devices in recent years,which demands for better flexible energy storage devices and promotes the development of flexible supercapacitors.Flexible supercapacitors can be divided into three types:planar,fiber-shaped and micro supercapacitors.Among them,the fiber-shaped supercapacitor?FSC?is the most promising one for wearable electronic devices due to its light weight,small size,flexibility on multiple dimensions and strong knittability.However,most reported FSCs have suffered low energy density which severely limits its practical application and development prospects.How to effectively enhance FSC's energy density while keeping its high power density is still a big challenge.Based on the formula:E=₂¹C(1²,the energy density?E?can be promoted by improving capacitance?C?or/and broadening the cell voltage?V?.C can be increased by developing advanced flexible fiber electrode materials and V can be expanded by constructing asymmetric supercapacitor.Pseudocapacitors can offer higher capacitance in comparison with the conventional electric double-layer capacitors?EDLCs?.Among various pseudocapacitive active materials,transition metal compounds,such as transition metal sulfide and phosphide,draw more attention than their corresponding metal oxide counterparts because of their high theoretical capacity,excellent conductivity and thermal stability.The research of this thesis mainly includes the following contens:?1?In this work,vertically aligned nickel cobalt sulfide?NiCo₂S₄?nanowires are grown on carbon nanotube?CNT?fibers using an in-situ two-step hydrothermal reaction method.The as-prepared NiCo₂S₄@CNT fiber electrode exhibits a high volumetric capacitance of 2332 F cm^-3,owing to its superior electric conductivity,large surface area,and rich Faradic redox reaction sites.Furthermore,a NiCo₂S₄@CNT//VN@CNT?vanadium nitride nanosheets grown on CNT fibers?asymmetric fiber-supercapacitor?AFSC?is successfully fabricated.The device exhibits an operating voltage up to 1.6 V,a high volumetric energy density of 30.64mWh cm^-3.The device also possesses outstanding flexibility,showing no obvious performance degradation under various bending angles and maintaining 91.94%capacitance after 5000 bending cycles under 90°.This work furnishes possibilities for advanced flexible wearable energy-storage devices.?2?A three dimensional?3D?nickel-cobalt phosphatized?NiCoP?nanowire based flexible fiber-shaped electrode were synthesized on carbon nanotube?CNT?fibers through a simple and economical method.The CNT fiber has high conductivity and small volume as the flexible current collector.This NiCoP nanowire array also exhibits excellent redox activity with high electrical conductivity and large effective surface area.The electrochemical performances of NiCoP@CNT fiber electrode was evaluated by cyclic voltammetry and galvanostatic charge and discharge tests.The results show that this NiCoP@CNT fiber electrode can achieve a high volume specific capacitance of 950 F cm^-3,which is greater than the corresponding nickel-cobalt oxide fiber electrode(NiCo₂O₄@CNT fiber,718 F cm^-3,prepared by hydrothermal synthesis and annealing treatment).What's more,it also delivers a high rate performance with a volumetric capacity of 685.5 F cm^-3 at 6 A cm^-3.This flexible NiCoP@CNT fiber electrode can be optimized to further improve its specific capacity and applied to construct flexible fiber-shaped asymmetric supercapacitors with high energy density and high power density.