Preparation Of NiMoO₄ Based Core-Shell Nanocomposites And Study On Properties Of Supercapacitors

In recent years,supercapacitors（also known as electrochemical capacitors）have attracted wide attention due to their advantages such as high energy density,good cycle stability,environmental friendliness and fast charge and discharge rate.More and more research groups are working on the design of novel pseudo-capacitor electrode materials for high performance supercapacitors.Transition metal oxides have good capacitive properties,especially binary transition metal oxides such as NiCo₂O₄,MnCo₂O₄,NiMoO₄,CoMoO₄ and MnMoO₄.Their different oxidation states and structures are capable of various REDOX reactions and are the most promising and low-cost pseudo capacitor materials.Among these binary metal oxides,NiMoO₄ has attracted extensive research interest due to its low cost,abundant natural resources,environmental friendliness and high theoretical capacitance.However,NiMoO₄’s low conductivity and low surface area usually exhibit limited kinetics in REDOX reactions.In addition,the pseudo-capacitance behavior of charge stored in the electrode material only occurs within a short diffusion distance of a few nanometers from the surface of the electrode material,leading to the unsatisfactory performance of the dual electrode asymmetric supercapacitor（ASC）.Therefore,developing and designing NiMoO₄-based electrode materials with high specific surface area and abundant active substances,trying to improve the specific capacitance of NiMoO₄-based electrode materials and improving the performance of NiMoO₄-based electrode materials are the main research contents of this paper.The specific research contents are as follows:（1）NiMoO₄@Ni（OH）₂ nanofibers with core-shell structure were successfully synthesized by electrospinning,high-temperature calcination and hydrothermal methods.Two kinds of capacitor materials,one-dimensional NiMoO₄ nanofibers and layered Ni（OH）₂ nanosheets,were combined to obtain the core/shell hierarchy,which enhanced the electrochemical performance.The prepared samples were electrochemical tested in A three-electrode system.The best core-shell structure NiMoO₄@Ni（OH）₂ nanofiber sample（NMH-2）showed a high specific capacitance（1293 F/g at 1A/g）and maintained 56.8%of the initial capacitance at a high current density of 10 A/g.The assembled two-electrode asymmetric supercapacitor NMH-2//AC also exhibits a high energy density of 46.8Wh/kg with a power density of 700 W/kg.The dual electrode asymmetric ultracapacitors have superior stability,maintaining 77.0%of the original capacitance after the cycle test（5000 cycles）and achieving 98.2%coulomb efficiency.The improved electrochemical performance is mainly attributed to the unique core-shell structure.The core of nanofibers is a one-dimensional structure that provides a stable and efficient way for ion and electron transport.The shell portion is a two-dimensional structure that greatly enhances the surface area and provides more surface active sites.In addition,the interconnected two-dimensional network structure makes the electrodes more stable during the cycle.In this homogeneous core/shell array,both"core"and"shell"electrochemical materials can be fully utilized and have a good synergistic effect.（2）In order to further improve the specific capacitance performance of core-shell NiMoO₄@Ni（OH）₂ nanofibers at constant current charge-discharge,the NMH-2 samples prepared in the previous stage were further optimized.One-dimensional NiMoO₄ nanofibers（NiMoO₄@Ni（OH）₂/Fe（OH）₃）were coated with Ni（OH）₂nanosheets and Fe（OH）₃ nanosheets by hydrothermal method.Three electrode system was used to test the obtained sample,the test results show that the specific capacitance is 1635 F/g.The obtained samples are assembled with asymmetric supercapacitors with two electrodes.The NMF//AC also shows a high energy density of 63.6Wh/kg when the power density is 700 W/kg.The dual electrode asymmetric ultracapacitors have superior stability,maintaining 70.1%of the original capacitance after the cycle test（3000 cycles）and coulomb efficiency up to 98.3%.The introduction of Fe（OH）₃nanosheets improves the specific capacitance performance of core-shell NiMoO₄@Ni（OH）₂ nanofibers.This work provides a reference for improving the electrode performance of core-shell structure NiMoO₄ based electrode materials.