Preparation And Electrochemical Energy Storage Of Transition Metal Layered Double Hydroxides Electrode Materials

Electrochemical energy storage is the most economical energy storage technology in the future energy industry.Developing novel electrode materials with high specific capacitance and excellent cycling stability,which is an effective approach to obtain high-performance electrochemical energy storage devices.As a representation of two-dimensional nanomaterials,layered double hydroxides（LDH）based on cobalt,iron,nickel,etc.are considered as promising candidates for superior electrode materials due to their abundant natural resources,tunable valence states and high redox activity in recent years.However,pristine LDH are suffered from finite active sites,poor electronic conductivity and structure stability,which severely limits their practical applications in the field of electrochemical energy storage.In this dissertation,the cobalt iron layered double hydroxides（CoFe LDH）were studied by constructing hierarchical nanostructures,doping metal ions and synthesizing composite materials by in situ sulfuration,the preparation technologies and optimization mechanism of electrode materials were discussed systematically.The CoFe LDH-based electrode materials with excellent electrochemical properties were developed,and asymmetric supercapacitors were assembled with different carbon-based materials.The main research contents and results are as follows:（1）The CoFe LDH nanosheets were directly grown on nickel foam（NF）substrate by a one-step hydrothermal synthesis without using any chemical binder.Subsequently,the Ni₃S₂/CoFe LDH/NF hybrid electrode with hierarchical structure was constructed via a facile electrodeposition treatment.The obtained electrode exhibits an areal capacitance of 5.08 F cm^-2at 2 m A cm^-2and an excellent rate capability of 73.8%capacitance retention when the current density increases to 20 m A cm^-2.After 8000 cycles,the areal capacitance holds a high retention of 89.8%and the morphology of electrode material can be maintained.The excellent electrochemical performance of the Ni₃S₂/CoFe LDH/NF electrode can be attributed to the synergetic contribution from Ni₃S₂and CoFe LDH active materials,providing rich redox reactions.Meanwhile,the hierarchical structure is beneficial to expose abundant electrochemical active sites and promote the acceleration of charge transport,leading to an enhanced conductivity.The asymmetric supercapacitor was assembled by the Ni₃S₂/CoFe LDH/NF anode and the activated carbon（AC）cathode,which shows a high energy density of 47.31 Wh kg^-1at a power density of 986 W kg^-1.Even after5000 cycles,the as-assembled device can hold 93.4%initial capacitance.（2）The Ni-CoFe LDH/NF electrode material with three-dimensional flower-like structure was prepared by a one-step hydrothermal method.The nucleation and growth of CoFe LDH crystal were controlled by Ni ions doping,and the three-dimensional flower-like structure was made from self-assembly of two-dimensional nanosheets.The as-obtained Ni-CoFe LDH/NF electrode shows an area capacity of 6.81 F cm^-2at a low current density of 2 m A cm^-2,as well as a good rate capability with 72.02%retention at a high current density of 40 m A cm^-2.After10000 charge/discharge cycles,it can maintain nearly 80%capacitance retention.Meanwhile,the Ni-CoFe LDH/NF electrode shows decreased overpotential of 129m V for the hydrogen evolution reaction（HER）at 50 m A cm^-2,which exhibits an outstanding electrocatalytic activity.The primary reason of excellent electrochemical properties for the Ni-CoFe LDH/NF electrode is the adjustment of microstructure,can result in a significant improvement on the electrochemical active area and redox activity.The effective contact between electrolyte and the active materials can increase and the transport distance of the electrolyte ions is reduced.（3）By use of a two-step hydrothermal synthesis,the CoFe LDH@Co₈FeS₈hybrid electrode material with three-dimensional-networked structure was developed via in situ sulfuration-induced strategy.The obtained binder-free electrode of CoFe LDH@Co₈FeS₈/NF exhibits a specific capacitance of 1515.28 F g^-1at 1 A g^-1and cycling stability with 75%capacitance retention after 6000 cycles at 10 A g^-1.The charge storage mechanism of CoFe LDH@Co₈FeS₈/NF electrode is predominantly contributed by the diffusive capacitance,also accompanying by surface-controlled capacitance behavior.The introduction of sulfur vacancies and synergistic effects of multiple active components can provide more active sites for electrochemical reactions,improving the charge storage and electron transfer ability of the electrode material.The asymmetric supercapacitor was assembled based on CoFe LDH@Co₈FeS₈/NF electrode and reduced graphene oxide（r GO）as negative electrode materials without using any organic binder,presenting an energy density of 37.16 Wh kg^-1at a power density of 841.35 W kg^-1,along with a wide potential window of 1.6 V.Additionally,it holds an outstanding stability performance of less than 3%capacity loss through 10000 continuous cycles.