Preparation And Electrochemical Performance Of Nickel-based Electrode Materials

Supercapacitors and nickel-metal hydride(NiMH)batteries are important energy storage devices,which are playing more and more important roles in the modern society.Supercapacitors possess high power density and long cycle life but low energy density and high cost.NiMH batteries exhibit many advantages,including high energy and power density,high current charge and discharge capability,high tolerance to overcharge and overdischarge,environmental friendliness,good safety,and low cost.However,no essential performance enhancements of the NiMH battery have been achieved for many years.In addition,the present supercapacitors and NiMH batteries show poor flexibility,let alone integrate them in a flexible system.Therefore,developing electrodes with low cost,high performance,and high flexibility is important for the development of supercapacitors and NiMH batteries.The research in this dissertation is based on the above problems.The main research content and results are as follows:The nickel(Ni)foam,which is used both as the current collector and reducing agent,is reacted with KMnO4 solution at room temperature.Ultrathin Ni(OH)2/MnO2 hybrid nanosheets with a thickness of 2-7 nm are grown on the surface of Ni foam vertically.Ni(OH)2 and MnO2 coexist in individual nanosheets in the form of small domains rather than forming different nanosheets,which may result from the unique reaction mechanism.In 1 M Na2SO4,the hybrid nanosheets obtain a specific capacitance of 723 F g-1 at a current density of 0.4 A g-1.In 1 M KOH and the mixed electrolyte of 1 M KOH+0.5 M Na2SO4,high values of 2325 and 2937 F g-1 are obtained respectively at a current density of 5 A g-1.The highest value is obtained in 1 M KOH+0.5 M Na2SO4 because of the synergistic effect of Ni(OH)2 and MnO2 domains.Solid-state symmetric supercapacitors are assembled using the Ni(OH)2/MnO2 hybrid nanosheets and PVA/KOH gel as the electrodes and electrolyte,respectively.An energy density of 21.5 W h kg-1 is obtained at a power density of 250 W kg-1.A high capacitance retention of 82.2%is obtained after 20000 charge/discharge cycles at 5 A g-1.Then,solid-state asymmetric supercapacitors are fabricated using the Ni(OH)2/MnO2 hybrid nanosheets and commercial activated carbon as the positive and negative electrode materials respectively.A high energy density of 91.13 W h kg-1 is obtained at a power density of 750 W kg-1.After 25000 charge/discharge cycles at 2 A g-1,a high capacitance retention of 92.28%is obtained.To enhance the energy density of the supercapacitors based on the whole device,we combine laser-beam drilling and electrodeposition to prepare ultrathin free-standing porous Ni films(PNFs)as the current collector.Through pores are drilled on a stainless steel sheet,which is subsequently filled with epoxy resin.The porous stainless steel sheet filled with epoxy resin in the pores is used as the template to prepare PNFs,whose thickness is as low as 2.5 μm and pore density is as high as 40000 pores cm-2.The specific surface area is 0.28 m2 g-1,which is higher than Ni foam and continuous nonporous Ni film.MnO2 nanosheets are electrodeposited on the PNF as the supercapacitor electrode.In three electrode system,the electrode shows a high specific capacitance of 2.8 F cm-2 at a current density of 3 mA cm-2,which is much higher than the electrodes prepared using Ni foam and continuous nonporous Ni film as the current collectors.After 5000 charge/discharge cycles at 15 mA cm-2,the electrode shows a high capacitance retention of 89%,which is much higher than the electrodes prepared using Ni foam(65%)and nonporous Ni film(31%).Then,solid-state asymmetric supercapacitors are fabricated using MnO2 and MoO3-x electrodeposited on the PNFs as the positive and negative electrode,respectively,and PVA/LiCl gel as the electrolyte,showing excellent flexibility.A high gravimetric energy density of 31.2 W h kg-1 is obtained at a power density of 285 W kg-1 based on the whole device weight including active materials,current collectors,electrolyte,and separator.In addition,a high volumetric energy density of 10.6 mW h cm-3 is obtained at a power density of 77.2 mW cm-3.The Ni matrix has a significant impact on the structure and performance of a NiMH battery.However,few studies have focused on the Ni matrix thus far due to the difficulty of fabricating controllable porous Ni materials.Here,we use silkscreen printing to print different insulating ink microarrays on stainless steel sheets,which are used as the templates to produce free-standing PNFs.The thickness of the obtained PNFs is as low as 2 μm.The pore size,density,and pattern structure of the PNFs can be easily controlled by changing the screen mesh structure(such as the mesh number,silk diameter,and silk spacing)and by employing a mobile platform for printing the templates.PNFs with pore sizes of 117,80,57,47,and 33 μm and pore densities of 1681,2831,5845,8309,and 14239 pores cm-2 are produced by using 150-,200-,300-,350-,and 450-mesh screen,respectively.Negative electrodes for the NiMH battery are prepared by using different PNFs as the Ni matrix and commercial hydrogen storage alloy(MmNi4.05Co0.45Mn0.38Al0.30X△)as the active materials,respectively.With the decrease of the pore size from 117 to 33 μm,the capacity increases firstly and then decreases,which reaches a maximum value of 214.7 mA h g-1 when the pore size is 47 μm.When using PNFs with the pore size of 47 μm,the capacity increases from 214.7 to 338.2 mA h g-1 as the pore density increases from 8309 to 25682 pores cm-2.Then,solid-state NiMH batteries are fabricated by using PNFs with the pore size of 47 μm and pore density of 25682 pores cm-2 as the Ni matrix and PVA/KOH gel as the electrolyte.MmNi4.05Co0.45Mn0.38Al0.30X△ andβ-Ni(OH)2 powders are used as the active materials for negative and positive electrode,respectively.The NiMH batteries show excellent flexibility and high energy densities of 151.8 W h kg-1(19.2 W kg-1)and 508.5 W h L-1(64.4 W L-1)as well as high energy efficiencies of 87.9-98.5%.The batteries outperform conventional NiMH batteries and many other commercial batteries,holding great promise for their future practical application.