Construction Of Porous MXenes Based On Pore Engineering St Rategy And Its Electrochemical Properties

In recent years,the gradual consumption of fossil energy and the environmental pollution caused by its combustion have brought a huge crisis to human society.In the context of"carbon neutrality"and"carbon peaking",the development of new energy sources has become extremely important.As a representative of electrochemical energy storage devices,supercapacitors are attracting more and more attention because of their high power density,fast charging and discharging,low cost and excellent cycling stability.Two-dimensional（2D）transition metal carbides,carbon nitrides and nitrides,or MXenes,not only possess common characteristics of 2D materials such as large specific surface area and anisotropy,but also have metal-like conductivity,strong hydrophilicity,large layer spacing and tunable surface chemistry,so MXenes show potential applications for supercapacitor active electrode materials.However,due to van der Waals forces,MXenes adjacent sheets are prone to self-stacking,resulting in slow ion transport and underutilization of active sites,which limits their practical applications.Constructing porous structures can effectively solve the above problems.However,due to the special etching preparation method of MXenes and the inherent defect of easy oxidation,the existing methods for constructing porous structures are lacking.Therefore,in this paper,the porous structure design is carried out based on the pore engineering strategy from the perspective of both in-plane and in-membrane pores to improve the electrochemical performance of MXenes electrodes based on the interlayer domain-limiting effect of MXenes.The specific research contents and main findings are as follows:（1）Construction of porous MXenes nanosheets based on microburst method and their electrochemical properties.According to the violent micro-explosion produced by the instantaneous gasification of liquid nitrogen molecules embedded in the accordion-shaped multilayer MXenes,the exfoliation of MXenes is assisted to realize the preparation of mesoporous nanosheets.The effects of MXenes etching conditions and processes on the preparation of mesoporous MXenes were investigated.Mesoporous Ti₃C₂T_xnanosheets were used as active materials for electrochemical energy storage to study their electrochemical properties.Due to the improved ion transport kinetics and the increase of active sites,the electrode exhibits a specific capacitance of 313.9 F g^-1at 1 A g^-1,which is 35.8%higher than that of the non-porous Ti₃C₂T_xelectrode.Excellent,with a capacity retention rate of 93.8%at 50 A g^-1.（2）Construction of mesoporous MXenes nanosheets based on thermal shock method and its electrochemical performance study.Based on the rapid sublimation and pyrolysis of n-butylamine molecules embedded between multilayer MXenes layers driven by hydrogen bonding,resulting in the exfoliation effect of the multiplication of air pressure in the interlayer confined space,a thermal shock-induced pore engineering strategy was developed to realize the mesoporous MXenes nanosheets.The effect of n-butylamine intercalation on mesoporous MXenes preparation was investigated.The electrochemical properties of mesoporous Ti₃C₂T_xnanosheets were investigated as electrochemical energy storage active materials.Ultra-high capacity(up to 494.8 F g^-1mass specific capacity at 5 m V s^-1,which is 82%higher than that of nonporous nanosheet electrodes,and up to 856.0 F cm^-3volume specific capacity)and excellent multiplicative performance(91.5%capacity retention at 2 000 m V s^-1)as well as cycling stability（98.5%of its capacity retention after 10 000 cycles）were obtained.Moreover,the capacity retention is close to 60%even at relatively high loading conditions(7.89 mg cm^-2).At the same time,the assembled symmetrical supercapacitor provides a high energy density of 11.1 Wh kg^-1at a power density of 250.6 W kg^-1.（3）Construction of porous MXenes thin films based on in situ foaming method and its electrochemical performance study.Based on the bulk effect of Si O₂microsphere hard templates and the accumulation of Si F₄gas in the restricted space within MXenes films after microsphere etching,an in-situ foaming strategy was developed to realize the preparation of porous MXenes films.The effect of Si O₂dosage,HF acid concentration and alkaline washing treatment on the preparation of porous MXenes films was investigated.composite films with 30%Si O₂dosage were etched with 30%concentration of HF acid and alkaline washed with KOH to obtain porous MXenes films with a specific capacity of 354.1 F g^-1at a sweep rate of 5 m V s^-1and a capacity retention rate of 85.1%(5～5000 m V s^-1).In addition,the electrode maintained 90.08%capacitance after 10000 cycles.Meanwhile,the assembled symmetrical supercapacitor provided a high energy density of9.05 Wh kg^-1at a power density of 848 W kg^-1.The research in this thesis provides new ideas for MXenes in-plane pore construction and MXenes electrode porous structure design,verifies the contribution of pore engineering strategies to increase ion transport and enhance electrochemical performance characteristics,and provides a theoretical basis for supercapacitor structure design.