Construction Of MXene And Its Derivative Hybrid Electrode And Their Capacitive Deionization Performance

Secure freshwater supply has become an increasingly significant problem due to the dramatic population increase and water quality deterioration.Therefore,sustainable freshwater resources are important scientific issues related to human development.Due to the large reserves of seawater on the earth,desalination of seawater or brackish water has become an important method to solve the crisis of shortage of freshwater resources.capacitive deionization（CDI）technology has attracted wide attention due to its low energy consumption,easy operation,and no secondary pollution.In recent years,inspired by electrode materials in the fields of ion batteries and supercapacitors,CDI electrode materials have expanded from electric double layer materials to faradaic electrode materials,and the desalination performance has been greatly improved.The tremendous characteristics of MXenes,such as high metallic conductivity,high surface area,ease of functionalization,activated metallic hydroxide sites,and hydrophilicity,make them consider to be a promising faradic cathode material in CDI.However,the layer self-stacking caused by van der Waals forces between layers restricts its further development and application as electrodes.This article focuses on the issues proposed above,and aims at high-performance MXene and its derived hybrid electrode materials.In this thesis,using electrode synthesis strategies such as intercalation,electrostatic induction,in-situ endogenous derivatization,in-situ exogenous growth,etc.,designed a full MXene flexible self-supporting membrane electrode with a larger interlayer spacing,a larger comparative area of three-dimensional porous alkalized MXene Membrane electrode,two-dimensional sodium titanate/graphene（M-NTO/rGO）composite membrane electrode,sodium titanium phosphate/graphene（M-NTO/rGO）composite electrode,and cobalt-based Prussian blue analogue/MXene composite electrode（CoHCF/M）,and these materials were applied in HCDI desalination system,with close investigation on their performance and mechanism.The main conclusions are stated as follows:（1）This chapter focuses on the problem of small interlayer spacing caused by easy stacking of MXene layers.Small size Ti₃C₂T_x（500 nm）MXene（S-Ti₃C₂T_x）nanosheets are used as intercalants,and large size Ti3C2Tx（lateral size≥1μm）is inserted.A full MXene（L-S-Ti₃C₂T_x）flexible membrane electrode is prepared between MXene（L-Ti₃C₂T_x）sheets,which effectively suppresses the stacking of MXene sheets and increases the layer spacing of the composite electrode.In the desalination test,it showed excellent desalination performance（72 mg Na Cl/g L-S-Ti₃C₂T_x,10 m M Na Cl solution）and cycle stability.This is due to the fact that the addition of the intercalator S-Ti₃C₂T_xshortens the Na ion transport path and provides more available active sites.This work proves the excellent performance and potential of MXene as an intercalant,and provides more possibilities for MXene to further expand its applications.（2）Considering the low capacity and poor cycle stability of the MXene material as the HCDI cathode material,the positively charged Na⁺ion is used to electrostatically induce the transformation of MXene into a pleated porous MXene（Alk-Ti₃C₂T_x）with a three-dimensional conductive network and a high specific surface area.Then,through a simple vacuum-assisted filtration method,the delamination Ti₃C₂T_x-MXene（d-Ti₃C₂T_x）and Alk-Ti₃C₂T_x are assembled into a binder-free Alk-Ti₃C₂T_x-M integrated electrode.d-Ti₃C₂T_x replaces the traditional conductive glue,can effectively adapt to volume changes,eliminates the electrochemical inert components of the traditional conductive glue,and enhances the capacity and stability of the electrode.In the HCDI test,the electrode has high capacity（50±3 mg/g at 30 m A/g）and long-cycle stability（～250 cycles,more than 10 days）.The electrode’s reasonable cost,low energy consumption and 15.3%energy recovery rate make it competitive in the industry.This assembled electrode strategy can be easily extended to other two-dimensional derivative materials in electrochemical applications.（3）Sodium titanate（NTO）has limited its application in CDI due to its poor conductivity,slow ion deintercalation reaction kinetics,and lattice expansion during charging and discharging.MXene can be used as a precursor for the synthesis of NTO because of its two-dimensional layered structure and abundant surface functional groups in this chapter,a two-dimensional sodium titanate/graphene（M-NTO/rGO）film material with titanium-MXene as the precursor and graphene as the constraining layer was prepared by solvothermal method.The NTO derived from Ti-MXene is sandwiched between rGO sheets to maintain a two-dimensional structure,which can alleviate the volume expansion of NTO during charging.In the HCDI desalination test,the desalination capacity can reach 57.57 mg/g（30 m A/g,±1.4 V）,the desalination rate is 0.019 mg/g/s,and the energy consumption is 0.42 k Wh/g.After 100desalination cycles,the desalination capacity remains stable.The excellent performance can be attributed to the membrane electrode.The ordered two-dimensional structure effectively suppresses interlayer stacking and provides more transmission paths and storage space for Na+ions and electrons.（4）NaTi₂（PO₄）₃（NTP）limits the reaction kinetics of ion deintercalation due to its inherent low conductivity and high contact resistance,thus hindering their application in CDI.This chapter uses a solvothermal synthesis method,using Ti-MXene as the precursor and graphene as the confinement layer derived synthetic M-NTP/rGO composite material.The synthesized product retains a part of MXene and constructs the MXene-NTP-rGO composite material system.In the desalination test,it showed a high capacity of 251.55 mg/g（current density of 30 m A/g,voltage of±1.8,10 Mm Na Cl solution）,low energy consumption of 0.19（working voltage of±1.0 V）,and long cycle stability（100cycles with a capacity of 147.5 mg/g）.The excellent performance of the electrode material can be attributed to the high conductivity provided by MXene and rGO.NTP inhibits the accumulation of graphene between rGO interlayers,increases the surface area and pore volume of the material,and provides more space and locations for the transmission and storage of Na⁺ions,And during the charging and discharging process,the graphene sheet layer can reduce the volume expansion of the NTP in the interlayer,thereby improving the cycle stability of the electrode.（5）Cobalt-based Prussian blue analog（CoHCF）was grown in situ using MXene as a conductive substrate,and a CoHCF/MXene（CoHCF/M）composite electrode material was successfully prepared.MXene can increase the conductivity of CoHCF and buffer its volume changes during charging and discharging.At the same time,CoHCF also effectively inhibited the stacking of MXene sheets.The composite electrode has a high specific surface area（374.29m²/g）,which can provide more transmission and storage space for Na⁺storage.Due to the synergy of the two,when CoHCF/M is used as an HCDI desalination electrode,it has excellent adsorption performance（103 mg/g）,low energy consumption（0.25 k Wh/kg）and cycle stability（100 cycles capacity retention Rate is 93%）.The research in this chapter shows the excellent performance and potential of MXene as a conductive substrate.The method is simple and versatile,and is expected to further expand its applications.