Synthesis And Electrochemical Performance Of Nickel Cobalt Oxides As Electrode Materials In Supercapcacitors

Supercapacitor has regarded as one of the most promising energy storage devices due to their long cycle life,fast charge and discharge rate,low maintenance cost,wide operating temperature range and friendly environment performance.Electrical double layer capacitor and pseudocapacitor have received extensive attention because of vari-ous advantages.However,in order to meet the growing demand for energy storage,hybrid capacitor devices could be regarded as a new chance,which is a bridge between the traditional supercapacitors and batteries and possess a higher energy density and faster the charge-discharge power,simultaneously.However,the cycle life and rate performance of the hybrid supercapacitor are still great challenges to make hybrid su-percapacitor more applicable.It is precisely the energy storage mechanism of battery-type materials which depended on the diffusion behavior not only on the surface but also in the bulk of active materials,crystal phase and the phase of the electrode mate-rials will change during the charge-discharge process and may result in significant vol-ume change of the morphology and then destroy the integrity of the electrode.To meet the main bottleneck of hybrid supercapacitor derived from the cathode materials,more reasonable atom ratio of nickel and cobalt was employed.In addition,by a facile hydrothermal method,the controllable morphology of Ni-Co-O could be obtained.The uniform morphology could make the electrode materials more tolerant during the charge-discharge process and then improve the cycle life of the hybrid de-vices.In addition,the nanostructure with narrow channel was designed and synthesized in order to minimize the mass transfer resistance in the diffusion process of electrolyte,and then improve the rate performance.Finally,in order to meet the urgent need of wearable electronic device,flexible electrode material and solid electrolyte were also employed and assembled to the supercapacitor device.The specific experimental pro-cesss and research results are exhibited,as follows.（1）In this part,urchin like Ni-Co-O nanospheres composed of nanowires struc-ture and flower-like Ni-Co-O nanospheres composed of nanosheets structure were suc-cessfully synthesized through a facile hydrothermal method.Moreover,by the addition of carbon fiber cloth during the hydrothermal process,the nanowire array and nanosheet array were also successfully grown on the carbon fiber.In briefly,due to the“3D-grid”structure composed by nanosheets could sustain larger structural alteration than nanowires structure during the repeated charge/discharge processes,the nanosheets array could exhibit better battery-type performance and much better cycling life.In addition,due to the avoiding use of polymer binder and conductive agents,it can be found that the Ni-Co-O nanosheets array and nanowire array directly grown on flexible carbon fiber cloth exhibits a lower internal resistance and better rate perfor-mance.As result,as prepared NCO-NSs/CFC exhibits higher capacity（425.4 mA h cm² at the current density of 1 mA cm²）as well as much better cycling stability（loss of³0%after 10000 cycles）The NCO-NSs/CFC was then fabricated into flexible all-solid-state symmetric device and delivered a maximum energy density of 92.4 Wh kg^-1 at a power density of 207.2 W kg^-1.This enhanced performance is attributed to several advanced structural factors of the NCO-NSs/CFC,including ultrathin and porous nanosheets,CFC support,and porous hierarchical structure.（2）The hierarchical walnut-like Ni-Co-O hollow nanospheres have been designed and successfully synthesized through a facile and effectual hard-template method.The outward-extensive sheets and hollow structure could be beneficial to the utilization of active sites as well as favorable electrolyte diffusion.Surprisingly,several layers with the thickness of 2-3 nm were found to be the main component of the outward-extensive sheets and then form the layered Ni-Co-O architecture.It is the ultrathin layers and the narrow-gap channel between them could not only provide abundant electrochemical active sites,but also shorten the diffusion length and reduce mass transfer resistance.As predicted,the walnut-like Ni-Co-O hollow nanospheres exhibits a large surface area(123.7 m² g^-1),superior specific capcity(221.8 mA h g^-1 at a current density of 1 A g^-1),rate performance(173.8 mA h g^-1 at a current density of 20 A g^-1)and excellent cycle life（the loss of specific capacity is only 0.6%after 3000 cycles）.Moreover,this battery-type material was assembled into typical hybrid device with commercial active carbon and the device exhibits superior energy density(38.3 W h kg^-1)and power den-sity(7604.9 W kg^-1).