Investigation On The Construction And Energy Storage Performance Of Nickel-cobalt-based Nanomaterials

With the increasing of global environmental pollution and energy consumption,the exploition of renewable new energy(e.g.wind power and solar energy)has made unprecedented progress in recent years.However,as renewable energy belongs to intermittent energy and sensitive to environment,it is difficult for the capacity to be directly incorporated into the power grid for efficient utilization.Therefore,the developments of efficient and safe energy storage technologies to achieve the maximum utilization of new energy have become a research hotspot.As an emerging energy storage technology,supercapacitors with favourable energy density and outstanding rapid charge-discharge performance can meet the energy supply requirements of portable intelligent electronic devices.Meanwhile,it can be applied as a power source for some large equipment such as new energy vehicles and submarine with the features of ultra-high power density and excellent operation safety.The design and development of supercapacitors with excellent performance are of great significance to the exploitation and utilization of new energy and the future progress of electronic market.We have designed and synthesized NiCo-based electrode materials with excellent morphology,excellent conductivity and outstanding stability from the perspective of two parallel design strategies of composition optimization and structure design,and applied them to flexible hybrid supercapacitors.The main research results are as follows:(1)A unique flexible Ni₂P/CoP nanoarrays self-supporting electrode with controllable size was synthesized by a facile and environmentally friendly method.The size controlled Nix Co_1-x(OH)₂ nanosheets array growing on the conductive substrate of flexible carbon cloth was successfully transformed into an ultra-thin,porous,self-supporting nano-network electrode.The excellent properties of the materials were analyzed by SEM,HRTEM,SAED,XPS,XRD and other testing methods in terms of morphology,lattice structure,crystal form and element valence.This three-dimensional porous network nanostructure can effectively increase the active sites and shorten the ionic transport paths,which is is favorable for charge transfer.Quasi-solid flexible hybrid supercapacitors with Ni-Co-P/CF as positive electrode and PVA-KOH hydrogel as electrolyte and separators deliver high power density and energy density(48.4 Wh kg^-1 at 811.2 W kg^-1),excellent electrochemical stability(88.8%capacity retention rate after 8000 cycles at 10 Ag^-1).After bending thousands of times at different angles,its capacity decay is almost negligible,showing the dependable mechanical stability and a promising practical performance.(2)Using the Kirkendall effect,hollow core-shell Ni-Co based sulphide nanomaterials combining the composition advantage and structure advantage were constructed.First,Ni-Co dihydroxide(NiCo-DHS)nanowires were supported on the surface of flexible carbon fiber cloth by hydrothermal method.Then,vertical ultrathin Ni(OH)₂ nanosheets were deposited on the surface of the nanowires by a simple electrodeposition strategy to yield NiCo-DHs@Ni(OH)₂ composite materials.Finally,hollow core-shell NiCo₂S₄@NiS composite materials with hollow core-shell structure were successfully prepared by using the Kirkendall effect in the tubular furnace.The advantageous physical/chemical properties of the NiCo₂S₄@NiS electrode material were analyzed from the aspects of morphology,structure,elemental composition and specific surface area by XRD,SEM,TEM,EDS,BET and other characterization methods.The synergistic effects of vertical monodisperse NiCo₂S₄ and surface NiS nanosheets accelerated charge transfer as well as improved redox reaction kinetics.In general,the infiltration range of electrolyte from the electrode surface to the inside is about 20 nm,meaning that materials with a depth of more than 20 nm have negligible contribution to the electrochemical energy storage.The construction of hollow structure effectively ameliorates this“dead volume”phenomenon and improves the utilization rate of electrode materials.The electrochemical properties of the composite structure were measured in a three-electrode system.It is illustrated that the composite structure holds lower electrochemical conductivity,excellent specific capacitance and cycling stability.The flexible wearable energy storage device was successfully assembled using NiCo₂S₄@NiS composite material as positive electrode,PVA-KOH hydrogel as electrolyte and separators,as well as activated carbon as negative electrode.The energy storage performance and practical application of the device were further confirmed.(3)Hollow dodecahedron of Ni-Co bimetallic selenide was synthesized for the first time using ZIF-67 as sacrificial template.As a sacrificial template,the metal-organic framework has been widely used in the synthesis of transition metal compound electrode materials.However,there are few reports on the application of MOFs templates in the synthesis of transition metal selenides.The reason is that most selenization processes are usually accompanied by high temperature reaction or strong corrosion,which is likely to lead to template collapse and structural damage.We used synthetic sodium selenihydride as selenium source to conduct anion exchange of ZIF-67 Mo Fs derived nickel-cobalt bimetal-hydroxide in a mild hydrothermal system,and successfully designed a hollow dodecahedral NiSe/CoSe₂electrode material with a large number of ultra-thin nanosheets on its surface.The bimetallic selenide delivers high specific capacitance and excellent capacity retention,which are far superior to the electrochemical properties of Co-Se and corresponding bimetallic hydroxide electrodes.The assembled quasi-solid flexible energy storage device has excellent electrochemical performance and mechanical flexibility.