Study On Structural Modulation Strategy For The Design And Construction Of Transition Metal Based Supercapacitor Materials And Their Electrochemical Properties

With the potential depletion of fossil fuels and the increasingly harsh environment caused by greenhouse gas emissions around the world,leading to an increasing demand for sustainable energy supply,there is an urgent need to develop a new-type energy storage device with low carbon and excellent performance.Supercapacitors（SCs）stand out among many electrochemical energy storage devices owing to their high output power,fast charge-discharge speed,long cycle life and safety,and have been widely employed in mobile communication,electric vehicles,and other fields.Nevertheless,the practical application prospect of SCs is still limited by their low energy density compared to batteries.Therefore,it is crucial to construct a reasonable morphological structure and utilize the synergistic effect between different components to improve the electrochemical properties of the composites and apply them to SCs electrode materials to further improve the energy storage performance of SCs.Transition metal compounds present a higher specific capacity than carbon materials and conducting polymers because of their abundant redox reaction sites and fast reversible Faraday redox reactions,so their research value in the field of electrode materials for SCs is extremely high.Based on the above elaboration,this paper designed a variety of transition metal compound based composite electrode materials with unique morphological structures through environmentally friendly and simple synthesis methods,mainly focusing on structural modulation strategies such as heterostructure design,conductive polymer modification and vacancy introduction to ameliorate their electrochemical performances,and then applied in SCs materials to improve energy density of the devices,the main research are as follows:（1）This work designs NiCo-LDH@MoO₃ composite material with unique heterogeneous structure.Herein,a unique intercalation pseudocapacitance characteristics and battery-type electrode materials layered double hydroxides（LDHs）nanosheet arrays are obtained by a simple and environmentally pollution-free twostep electrodeposition technology.The electrode materials are composed of MoO₃ and NiCo-layered double hydroxides（NiCo-LDH）,which is directly grown on the three-dimensional（3D）conductive nickel foam（NF）substrate,forming a binder free two-dimensional（2D）ultra-thin cross-multilayer heterostructure（NiCo-LDH@MoO₃）.The heterogeneous NiCo-LDH@MoO₃ nanomaterial manifests a prominent specific capacity of 952.2 C g^-1 at the current density of 1 A g^-1,and delivers good cycling performance of 86.42%capacity retention at current density of 20 A g^-1 after 10,000 cycles.Remarkably,the fabricated NiCo-LDH@MoO₃//AC HSC equipment exhibits an excellent energy density,whose value is 58.06 Wh kg^-1 with the power density of 800 W kg^-1,and the capacity retention rate and coulombic efficiency can reach 73%and 96.7%at 15 A g^-1,respectively,suggesting that it has application prospects in supercapacitor electrode materials.（2）Rationally constructing hydrangea-like Zn Co₂O₄/NiCo Ga-LDH@PPy composite material with core-shell heterostructure（denoted as ZCO/NCG-LDH@PPy）.Specifically,the ZCO/NCG-LDH@PPy employs ZCO nanoneedles clusters with large open void space and rough surfaces as the core,and NCG-LDH@PPy composite as the shell,comprising hexagonal NCG-LDH nanosheets with rich active surface area,and conductive PPy films with different thicknesses.Meanwhile,density functional theory（DFT）calculations authenticate the charge redistribution at the heterointerfaces between ZCO and NCG-LDH phases.Benefiting from the abundant heterointerfaces and synergistic effect among different active components,the ZCO/NCG-LDH@PPy electrode acquires an extraordinary specific capacity of 381.4 m Ah g^-1at 1 A g^-1,along with outstanding cycling stability（89.83%capacity retention）after 10,000cycles at 20 A g^-1.Furthermore,the prepared ZCO/NCG-LDH@PPy//AC hybrid supercapacitor（HSC）exhibits a remarkable energy density(81.9 Wh kg^-1),an outstanding power density(17,003.7 W kg^-1),and superior cycling performance（a capacitance retention of 88.41%and a coulombic efficiency of 93.97%）at the end of the 10,000th cycle.Finally,two HSCs in series can light up a LED diode for 15 min,indicating its excellent application prospects.（3）Ni₃S₂/Co₉S₈@（Co,Mn）Se₂ composite material with rich Se vacancies（NCS@Vr-CMSe）is prepared by simple hydrothermal,electrodeposition technologies and Na BH₄reduction treatment method.The electrochemical test results show that when the current density is 1 A g^-1,NCS@Vr-CMSe delivers excellent specific capacity performance of 1134 C g^-1,meanwhile,it still has an excellent specific capacity retention of 90.2%after 10000 GCD cycles.The assembled NCS@Vr-CMSe//AC hybrid supercapacitor（HSC）displays a high energy density of 79.5 Wh kg^-1 at a power density of 799.9 W kg^-1,with the capacitance retention of92.81%and coulombic efficiency of 98.95%after 10000 charge-discharge cycles.Finally,two HSC devices in series can illuminate a LED diode for up to 23 min.The research demonstrates that the introduction of heterointerfaces and selenium vacancies can improve the electron transfer rate,produce atom rearrangement,and increase the active sites on the electrode surface,thus improving the electrochemical performance such as conductivity and specific capacity.