Constrction Of MXene-Based Film Electrode And Its Capacitive Deionization Performance Investigation

In recent years,with the continuous growth of population and rapid economic development,human demand for fresh water resources has increased sharply.In addition,the continuous expansion of cities and the rapid progress of industrialization have resulted in a large amount of water pollution,further aggravating the shortage of fresh water resources.Therefore,it is of vital significance for human survival and development to solve the problem of shortage of fresh water resources.The earth is rich in seawater and brackish water,so desalination of seawater or brackish water is an effective way to solve the shortage of fresh water.Capacitive deionization（CDI）technology has become the most promising desalination technology due to its advantages of low energy consumption,simple device and no secondary pollution.The material of CDI electrode has an important influence on its desalination performance.However,the adsorption capacity of traditional non-Faraday electrode materials（mainly carbon-based materials）is low,which limits the application of CDI technology in practical production.Two-dimensional material MXene has good conductivity and hydrophilicity,and has excellent pseudo-capacitance characteristics and electrochemical stability,which makes it a rising star of CDI electrode materials.However,there are still some problems in the research of CDI technology based on MXene electrode.Firstly,due to the existence of interlayer van der Waals forces,MXene nanosheets tend to self-stack during electrode preparation,which weakens the ion storage capacity of MXene electrodes.Secondly,the adsorption capacity of LiF/HCl etched Ti₃C₂ Tx MXene as anode is relatively low due to the presence of-O,-F or-OH terminal groups on the nanosheets surface.Third,the current CDI based on MXene electrode mainly consists of symmetric electrode CDI and hybrid electrochemical deionization（HCDI）,but it lacks research in the field of dual-ion electrochemical deionization（DEDI）.To address the above problems,this paper aimed to obtain high performance MXene composite flexible film electrode,taking Ti₃C₂ Tx MXene etched by LiF/HCl as the main material,first explored the desalination behavior of pure MXene film electrode,and then to solve the problem of MXene self-stacking and insufficient adsorption capacity when acted as anode.MXene-based film electrodes with high desalination performance were prepared by in-situ and ex-situ composite method.It was applied to symmetrical electrode CDI,HCDI and DEDI systems to test the desalination performance and explore the desalting mechanism.The specific works are as follows:（1）MXene was prepared by LiF/HCl etching method,the flexible film was obtained by vacuum filtration,and the symmetric MXene CDI system was assembled.The desalination conditions under constant current / constant pressure working mode was investigated respectively,and the mechanism was analyzed.Then the influence of the introduction of ion exchange membrane on the desalination performance of the MXene CDI system was further investigated.The results showed that in MXene CDI system,it is possible to obtain a desalination process that is consistent with conventional CDI or an inconsistent process that is called "desalination generator" by changing the applied potential direction.In addition,it was found that the MXene CDI system without ion exchange film adsorbed salt twice and desorbed salt twice in a desalination cycle（± 1.2 V voltage range）.And the introduction of ion exchange membrane reduced the energy consumption of the device,obtaining higher energy recovery rate and desalination rate.（2）In order to solve the self-stacking problem of MXene electrode,zerodimensional carbon dots（CDs）ware used as the interlayer spacer of MXene nanosheets.Positively charged zero-dimensional CDs adsorbed on the surface of negatively charged two-dimensional MXene nanosheets to form CDs@MXene micro-flowers,which weakened the self-stacking of MXene nanosheets and increased the specific surface area of the electrode materials.At the same time,CDs with small particle size did not affect the flexibility of MXene,and CDs@MXene could still form a flexible composite film.Electrochemical analysis showed that the CDs@MXene electrode had good electrochemical performance,and its surface charge control contribution capacity ratio was higher than that of pure MXene electrode.The system was assembled into a symmetrical CDs@MXene CDI system,and a desalination capacity of 86.4 mg/g（10m M NaCl,20 m A/g,±1.2 V voltage range）was obtained with good cyclic stability.（3）In order to solve the problem of low adsorption capacity of chloride ions when MXene was used as anode,Bi material with electrochemical absorption chloride ion ability was introduced into MXene nanosheets,and Bi@MXene composite film was prepared in situ.The content of Bi in Bi@MXene composite film reached 5.3 wt %,and was evenly distributed in the entire MXene layered structure.Electrochemical analysis showed that the composite film had dual mode ion storage behavior.The composite film and activated carbon electrode were assembled into AC ∥ Bi@MXene HCDI system.The desalination capacity of 61.0 mg/g（20 m/g,± 1.2V）was obtained in 10 m M NaCl solution.The desalination rate of 4.2 mg/g/min was obtained at 200 m A/g current density.The desalination performance of the system under different salt concentrations was tested.It was found that the desalination capacity reached 100.5mg/g in 30 m M NaCl solution.In addition,the cyclic stability performance of the system was tested.In 40 cyclic desalination tests,the system did not show the usual capacity decay,but increased by 45.8%.（4）In the previous chapter,we proved that the strategy of preparing composite electrode by in-situ introducing Bi into MXene film was effective in improving the desalination performance of electrode.However,MXene had limited adsorption capacity for Bi³⁺.Therefore,in this chapter,Bi₂O₃ was directly introduced into MXene to obtain Bi₂O₃@MXene composite film,which could be used as the Faraday electrode of sodium ion.In addition,after the reduction of the Bi₂O₃@MXene film with NaBH₄,the Bi@Mene composite film used as the Faraday electrode of chloride ion.The Faraday electrode of chloride ion and sodium ion could be obtained by a simple vacuum filtration process.The two composite films were assembled into Bi₂O₃@MXene ∥Bi@MXene DEDI system.The desalination capacity of 86.8 mg/g（20 m A/g,±1.2 V voltage range）and the desalination rate of 3.25 mg/g（200 m A/g）were obtained.In addition,by connecting the two DEDI devices in series,they could be used as a new power source when the salt was desorpted,and the stored energy turned on an LED light for more than five minutes.