Study On Preparation And Supercapacitor Performance Of Nickel/cobalt-based Composite For Positive Materials

Supercapacitors(SCs),as a class of energy conversion device,have been widely applied in the field of electronic products,new energy vehicles and micro-grid due to their fast charge and discharge process,high power density and high cycling stability.However,most of the used SCs show insufficient in terms of the specific capacitance and energy density.Hybrid supercapacitors are typically composed of a faradic-type electrode and a double electric layer capacitor-type electrode.Such a hybrid structure can take full advantages of two sides,from which a useful supercapacitor with high electrochemical performance can be obtained.Nickel/cobalt-based materials with high theoretical specific capacitance are chosen as the faradic-type electrodes for hybrid supercapacitor.This article aims at the fabrication of series of three-dimensional nickel/cobalt-based hydroxides,sulfides/phosphids and the composite materials by different synthesis strategy,and look for materials with high specific capacitance and high energy density for hybrid supercapacitors.A flower-like Ag@nickel-cobalt layered double hydroxide(Ag@NCLDH)composite was synthesized via one-step solvothermal method without any template.Compared to pure NCLDH,the Ag@NCLDH exhibited more expanding hierarchical structure,which was in favor of the contact between electrode and electrolyte.Benefiting from the unique structural features,these flower-like Ag@NCLDH microspheres showed impressive specific capacitance as high as 1768 F/g at 1 A/g.And it still delivered 68.8%of the initial specific capacitance at 10 A/g.After 1500 cycles of charge and discharge process,76.5%of the initial specific capacitance still preserved.The Ag@NCLDH electrode exhibited superior electroactive than pure NCLDH.Nickel/cobalt layered double hydroxides with unique 3D tube structure were first synthesized via solvothermal method.Subsequently,it was chosen as the precursor for the preparation of nickel cobalt sulfides(NCS)with a hydrothermal process.The tube structure was retained after the sulfuration reaction,though the tube size of NCS was smaller than the precursor.In virtue of the special structural merits,these NCS microtubes have an impressive specific capacitance as high as 1148 F/g at 1 A/g and excellent rate capability(83.1%at 10 A/g).The specific capacitance of NCS electrode still remains 75%of its initial value after continuous cycling for 2500 cycles.Metal-organic frameworks(MOFs),as kind of ordered and porous materials,always possess higher surface area and abundant porous structure than most inorganic materials.As reported by previous work,MOFs can be used as template for the acquiring of high performance materials.Here,xrGO/Ni2P composites are successfully synthesized via an In-situ phosphorization process with GO/Ni-MOF as precursors.Compared to pure Ni2P,the xrGO/Ni2P composites appear enhanced electrochemical properties in terms of the specific capacitance and cycling performance as electrodes for supercapacitors.Especially,the 2rGO/Ni2P electrode shows a highest specific capacitance of 890 F/g at 1 A/g among the obtained composites.About 54.4%of the initial specific capacitance was retained at 10A/g.And the specific capacitance of the 2rGO/Ni2P based electrode still maintains 61.1%of its initial value after 4000 cycles.The results demonstrate that MOFs derived materials are potential materials for supercapactiors.ZIF-67(zeolite imidazolate frameworks material)combained with cobalt ions and methyl imidazole has ordered polyhedral structure,and it can be easily removed under acid solutions.On the basis of the feature,nickel nitrate was selected as the acidic medium,and ZIF-67 particles were etched under a facile solvothermal process.The structure and morphology of materials synthesized with different ration of nickel nitrate were investigated.By controlling the addition amount of nickel nitrate,nickel/cobalt hydroxides with 3D cages structure were obtained.Owing to the unique hollow structure,high surface area and mesoporous structure,NCLDH-3 electrode exhibited a highest specific capacitance of 1616 F/g at 1 A/g among the obtained materials.It still delivered a specific capacitance of 1376 F/g at 10 A/g,indicating an excellent rate capability.After tested with 4000 cycles,about 63.1%of the initial value was preserved.In order to further enhance the specific capacitance and cycling performance of NCLDH cages derived from ZIF-67,surface modified carbon nanotubes were introduced during the preparation of ZIF-67.A cross-linked ZIF-67@CNTs composite was obtained by the synthesis strategy.Subsequently,the cross-linked ZIF-67@CNTs was treated under the optimized condition discovered above.It is worth mentioning that the surface of the exposed carbon nanotubes was covered by nickel/cobalt hydroxides nanosheets.The electrochemical measurement results showed that NCLDH@CNTs composite possess a high specific capacitance of 1833.6 F/g at 1 A/g,and about 82.7%of the value was reserved at 10 A/g.Besides,after continuous cycling for 4000 cycles,the NCLDH@CNTs electrode still retained 79.0%of its initial specific capacitance.The assembled NCLDH@CNTs//AC hybrid supercapacitor exhibited a superior energy density of 32.5 Wh/kg at a power density of 150 W/kg,and still retained 15.0 Wh/kg at 6584 W/kg.The NCLDH@CNTs//AC HSC device also showed favorable cycling performance and approximately 78.7%of its initial specific capacitance was still preserved after 8000 cycles.And the HSC device was applied to drive a mini fan and lighten the LED bulb,which showed great potential in practical application.