Preparation And Electrochemical Energy Storage Behavior Of Conductive Metal Organic Frameworks

As a new type of two-dimensional material,2D c-MOFs are rapidly becoming a hot topic of research in the field of electrochemical energy storage as they exhibit good electrical conductivity,large specific surface area,regular and rich pore structure and environmental friendliness.Its high electrical conductivity facilitates fast electron transfer to redox active sites,thus enabling fast charging and discharging;its large specific surface area and open and regular channels facilitate ion de-embedding;its rich active sites provide higher capacity and energy density;and its rigid and extendable framework structure is more stable during repeated ion de-embedding.However,2D c-MOFs for electrochemical energy storage electrode materials are still very limited,and the specific roles of metal centres and organic ligands on their physicochemical properties and electrochemical performance are still unclear.Therefore,the aim of this thesis is to develop and explore stable and efficient 2D c-MOFs electrode materials,and to optimise the design ideas and synthetic routes of MOFs-based electrode materials to investigate their electrochemical properties and energy storage mechanisms in detail.The specific work is as follows:（1）A novel,structurally stable 2D c-MOFs material with good electronic conductivity,zinc-benzene hexathiol（Zn-BHT）,was synthesized and its electrochemical properties in terms of zinc storage were investigated in detail.Each sulfur atom of BHT was bound to two Zn²⁺to obtain an extendedπ-d conjugated structure in the 2D plane,with the 2D planes stacked together at 0.35 nm layer Zn²⁺can insert/detach from the host structure at a high diffusion rate,exhibiting a pseudocapacitance mechanism with fast kinetics.At 100 m A g^–1,Zn-BHT exhibited a reversible discharge capacity close to 90.4 m Ah g^–1,good multiplicative performance,and cycling stability.In addition,the constant current intermittent titration（GITT）test allows us to conclude that Zn-BHT also has very fast Zn²⁺diffusion kinetics,indicating that Zn-BHT has good zinc ion transport properties.Furthermore,ZIC devices were assembled using a Zn-BHT cathode and polyaniline-derived porous carbon（PC）as the anode,which had a potential window of up to 1.8 V and a high energy density of about 37.2 Wh kg^–1.This work shows that conducting MOFs are promising ZIC cathode materials and provides new ideas for the design and development of electrode materials for application in electrochemical energy storage（EES）devices.（2）To address the problems of low power density and poor cycling stability of lithium-ion capacitors（LIC）due to mismatched positive and negative kinetic processes,we have synthesized Zn₃（HHTP）₂（HHTP=2,3,6,7,10,11-hexahydroxytriphenylene）MOF materials with good electrical conductivity and rich pore structure by hydrothermal methods and investigated their lithium storage performance as LIC negative electrodes in detail.Due to its unique structure,Zn₃（HHTP）₂ was able to achieve a discharge specific capacity of 429 m Ah g^–1 after 1000 cycles at a high current density of 1000 m A g^–1,and has a good multiplicative performance.XPS tests after cycling revealed no significant change in the valence state of the metal ions,and it is speculated that the metal ions may not be involved in the lithium storage process,with the lithium storage sites including organic ligands,pore structures,and interlayers.In addition,Zn₃（HHTP）₂ has fast Li⁺diffusion kinetics with Li-ion diffusion coefficients between 10^-12-10^-9 during charging and 10^-11-10^-8 during discharging,indicating that Zn₃（HHTP）₂ has good Li-ion transport properties and can effectively alleviate the LIC kinetic mismatch.The LIC device was constructed using Zn₃（HHTP）₂ as the negative electrode and activated carbon as the positive electrode,with an energy density of～104.8 Wh kg^–1 at a power density of 400 W kg^–1,and an energy density of～68 Wh kg^–1 even at a high power density of 2k W kg^–1.（3）In this chapter,Cu（OH）₂ nanowire arrays（Cu（OH）₂@CF）were grown in situ on the surface of copper foam using a rapid redox method,and then Cu（OH）₂@CF was immersed in an aqueous solution of HHTP ligand to obtain a Cu₃（HHTP）₂@CF self-supporting electrode material by HHTP and OH^–ion exchange to obtain Cu₃（HHTP）₂@CF self-supporting electrode material,which is directly used in the negative electrode of Li-ion batteries.The nanoscale array structure not only facilitates the full exposure of the active sites and the penetration of the electrolyte,but also the voids between the arrays greatly alleviate the volume changes during charging and discharging.The Cu₃（HHTP）₂@CF self-supported electrode material with a reaction time of 4 h achieves a discharge specific capacity of 1.83 m Ah cm^–2 for the first turn at a current of 1 m A,and also exhibited excellent multiplicative performance,returning to 90%of its initial value when the current was restored from 10 m A to a small current of 1 m A.And Li⁺can insert/detach from the host structure at a high diffusion rate,exhibiting a pseudocapacitance mechanism with faster kinetics.