Preparation And Electrochemical Properties Of Nickel-cobalt-based Porous Electrode Materials

Supercapacitors have the characteristics of fast charging and discharging and long cycle life,which meet the current demand of high-performance energy storage equipment.The development and application of electrode materials which are the key components of capacitors have become a research hotspot at present.Pseudocapacitive materials have become the focus in research of energy storage due to their fast Faradaic reversible redox reaction and ion exchanges,resulting in high theorical capacity and energy density.However,poor conductivity and low specific surface area seriously limit the performance of the electrode at high current density.One of the strategies is to build a multi-metal compound by using the synergistic effect of variety of transition metal,or to design a core-shell structure coated with conductive polymers to effectively expand the conductivity of the materials.Another method is to reasonably design porous nanostructure with high specific surface area to reduce stress and volume changes,thus providing more active sites and reducing the transmission distance between electrode and electrolyte.Therefore,the design and synthesis of multi metallic composite with high specific surface area is the key to the development of supercapacitors with high energy storage performance.In this paper,the core-shell nanocomposites were synthesized by coating spinel NiCo₂O₄with conductive polymers,and the hollow nanostructure of porous multi metallic salt were designed.The effects of coating thickness and metal ratio were researched on the microstructure and electrochemical properties of the prepared electrode.The main work is as follows:(1)The NiCo₂(OH)₆ precursors with different dimensional structure were in-situ synthesized by hydrothermal method assisted by different surfactants on carbon cloth,and then porous NiCo₂O₄ nanomaterials were obtained by high temperature calcination.Different surfactants act as soft templates to control the direction of crystal growth and synthesize porous structures with different dimensional.Among them,sodium dodecyl sulfate(SDS)assisted synthesis of NiCo₂O₄ nanowire arrays lengthening about 500 nm with high specific surface area(113.2 m²g^-1)provide more active sites,thus shortening the diffusion distance of electrolyte,and improving the capacity.Electrochemical performances show that compared with NiCo₂O₄/CC-PVP and NiCo₂O₄-CTAB electrodes,the NiCo₂O₄/CC-SDS electrode has higher capacity(103.1 m A h g^-1 at current density of 1 m A cm^-2)and excellent rate performance(81%capacitance in the range of 1?10 m A cm^-2).In addition,NiCo₂O₄ prepared with high concentrations of SDS can form nanoflower network with high specific surface area,but it shows poor electrochemical performance.(2)Porous NiCo₂O₄ nanowire arrays were in situ grown on carbon cloth and then NiCo₂O₄@Ppy nanomaterials with core-shell structure prepared by galvanostatic electrodeposition of polypyrrole(Ppy).It is the first time to use polyacrylic acid sodium as impurity agent to prevent the anion exchange between electrolyte and polypyrrole during charge and discharge,showing high structural stability and long cycle life.In addition,a suitable and thin polypyrrole layer(8-40 nm)on NiCo₂O₄ material forming core-shell structure with high specific surface area(131.7 m² g^-1),uses the synergistic effect of NiCo₂O₄and polypyrrole,thus reducing the transfer resistance of electrolyte ions in the electrode materials and showing better electrochemical properties.Comparing with NiCo₂O₄@Ppy-5s/CC and NiCo₂O₄@Ppy-15s/CC,NiCo₂O₄@Ppy-10s/CC electrode exhibits higher specific capacity(155.4 m A h g^-1 at 1 m A cm^-2),excellent rate performance(88%at 10 m A cm^-2)and long cycle life(71%after 8000 cycles).In addition,assembled an all-solid state hybrid supercapacitor(NiCo₂O₄@Ppy-10s/CC as positive electrodes)exhibits high energy density(22.3 W h kg^-1)at a power density of 800 W kg^-1.(3)The 3D porous Ni_xCo_2-x(OH)₃Cl hollow nanomaterials were simply synthesized by deep eutectic solvent(DES)assisted coprecipitation method reported by first time.Preparation of new multi-metal compounds is to use the synergistic effect between different metals to improve the efficiency of electron transfer.The porous hollow structure can accelerate the diffusion rate of electrolyte ions in Ni_xCo_2-x(OH)₃Cl nanomaterials,reducing the reaction temperature and time effectively by using DES,which can be used in large-scale production.Electrochemical performances show that Ni_0.5Co_1.5(OH)₃Cl electrode has high specific capacity(225.3 m A h g^-1 at 1 A g^-1),excellent rate performance(146.1 m A h g^-1 at 20 A g^-1),and capacity retention rate was 64%at 5 A g^-1 after 5000 cycles.The electrochemical performance of Ni_0.5Co_1.5(OH)₃Cl electrode in Li OH electrolyte is better than that in Na OH and KOH solution.In addition,a hybrid capacitor(Ni_0.5Co_1.5(OH)₃Cl electrode as cathode)has high energy density(50.62 W h kg^-1 at power density of 850.2 W kg^-1)and specific capacitance retention rate of 75.3% after 10,000 cycles.