Construction Of LDHs Based Flexible Electrodes And Study On Their Supercapacitor Performance

Electrochemical capacitors(or named supercapacitors)have attracted considerable attentions in electric vehicle,aircraft and personal multimedia,due to their large capacitance,high power density and long cycling life.In recent years,flexible and wearable microdevices play an increasingly important role in both industry and human life.To drive such devices,the power supplies should be lightweight,flexible and tiny in volume.Fiber-shaped supercapacitors(FSSCs)are regarded as a promising power source for flexible/wearable devices based on these advantageous characteristics.Using carbon based materials to develop FSSCs is the most common method.However,the capacity of such FSSCs is rather low because of their inherent double-layer charge-storage mechanism.In addition,another limitation of these carbon-based FSSCs is their small electrolyte-accessible surface area:only the outer surface of the fibers is accessible,leading to disability for ion transmission into the fiber electrodes and thus an unsatisfied capacity.In order to solve these problems,the performance of supercapacitor is expected to be improved by incorporating pseudopotential materials with high electrical conductive fiber.Transition metal layered double metal hydroxide(LDH)is kind of important pseudocapacitor materials,which have the advantages of high electrochemical activity,good cycling stability and simple preparation and so on.In this work,CoNi-LDH and NiO@LDH arrays are grown on the surface of hollow RGO fiber and Ni wire;and the supercapacitor performance is enhanced by controlling its composition and structure.The microsupercapacitor is further constructed showing higher energy density and cycle stability.The specific results are shown as follows:(1)Construction of RGO@CoNi-LDH fiber-shaped flexible electrodes and study on their supercapacitor performance.Firstly,the layered stacked hollow RGO fiber was prepared by hydrothermal method and then CoNi-LDH nanosheets(size?500 nm and thickness?10 nm)are grown on the inner and outer surfaces of hollow RGO fiber by electrodeposition method.The obtained RGO@CoNi-LDH electrode exhibits a specific areal capacitances up to 570 mF cm-2(at 0.25 mA cm-2),as well as long term cyclability(over 95.3%capacitance retention after 2000 cycles at 1 mA cm-2).The excellent supercapacitor performance is ascribed to a synergistic effect among the hierarchical core-shell nanostructure,good conductivity of RGO and high pseudocapacitve activity of CoNi-LDH(size 80-100 nm and thickness?10 nm).Benefiting from their robust mechanical property and bending durability,such RGO@CoNi-LDH fiber electrodes are capable of being woven into a wearable fabric.The RGO@CoNi-LDH fiber electrode and RGO@AC electrode were further assembled into an all-solid-state supercapacitor,which exhibits superior energy and power output,with energy density of 8.89 ?Wh cm-2 at a power density of 0.525 mW cm-2.The energy and power densities are in the highest level among those of the reported FSSCs.It is expected the hollow RGO@CoNi-LDH based FSSCs can be potentially used as highly efficient energy storage device in the flexible and wearable electronics.(2)Construction of Ni/NiO@LDH fiber-shaped flexible electrodes and study on their supercapacitor performance.Hybrid Ni/NiO@LDH fibers consisting of porous NiO(width 0.5-1?m,length 6-8 ?m,thickness?20 nm)as core and LDH nanoplate array as shell layer have been successfully prepared via a internal source etching and electrochemical electrodeposition method.The obtained Ni/NiO@LDH electrode exhibits a high specific areal capacitances(1.63 F cm-2 at 2.5 A cm-2),good rate performance(the retention ratio was 90.7%when the current density increased from 2.5 A cm-2 to 20 A cm-2)and high Coulombic efficiency(?100%),retention ratio up to 97.6%remains after 1000 cyclic test.The Ni/NiO@LDH fiber electrode and Ni@AC electrode was further assembled into an all-solid-state supercapacitor,which exhibits superior energy and power output.When the power density increases from 150 mW cm-3 to 900 mW cm-3,the energy density of the device reduces from 6.98 mWh cm-3 to 6.0 mWh cm-3.The energy and power densities are in the highest level among those of the reported FSSCs.Compared with the RGO@CoNi-LDH hollow fiber electrode,Ni/NiO@LDH electrode has shows better electrochemical performance(in view of specific capacity,rate performance,cyclic stability).Moreover,the fiber supercapacitor fabricated by Ni/NiO@LDH electrode has higher energy density and power density output than RGO@CoNi-LDH//RGO@AC microsupercapacitor.