Surface Regulation In MXene Electrodes For Capacitive Deionization

The scarcity of fresh water resources and the utilization of salt water resources are inseparable from desalting technology.There are many technologies to obtain fresh water and metal salt resources by desalting seawater or brackish water.Capacitive Deionization（CDI）causes dissociation of salt ion by means of power failure or electrode reverse connection,contributing to electrode regeneration and realizing electrode recycling,which has its unique advantage in desalting technology.It has gradually become a promising technical means to alleviate the shortage of fresh water resources,water pollution and metal salt resource recovery.In recent years,CDI processing technology is moving towards the direction of high efficiency electrode and no secondary pollution.In the future,important directions such as functional carbon-based electrode materials（carbon materials,titanium carbide MXenes,doped modified graphene）will be further focused.Layered MXenes materials with adjustable surface functional groups offer the possibility of efficient ion transport.More importantly,this kind of material has the advantages of high conductivity,good hydrophilicity,and easy to bond with metal ions,so it has greater advantages as a CDI electrode.However,the surface-F terminating groups of Ti₃C₂T_xMXenes prepared by traditional highly corrosive fluorine-containing etching significantly hinder ion transport and charge transfer.In order to further study the MXenes-based CDI desalination technology and explore its potential in practical applications,two surface structure regulation methods were proposed in this study,respectively studying the material surface structure and desalting mechanism.Specific research contents include:（1）In this study,an electrochemical etching method of three-electrode system in alkaline environment was proposed to synthesize a new type of Ti₃C₂T_xelectroadsorption material（including-OH,-O）with no-F terminating group on the surface.Better hydrophilicity is conducive to ion transport in aqueous system,improving the adsorption capacity and adsorption rate of CDI,and its salt removal capacity is as high as 20.27 mg·g^-1.The adsorption rate was 1.01 mg·g^-1min^-1.Moreover,the secondary current generated by the desorption process can light up LED bulbs,promising future applications for CDI in desalination and renewable energy.（2）The self-film-forming Ti₃C₂T_xwithout binder has a larger theoretical capacitance.However,due to the limitation of the flexible membrane prepared by vacuum filtration,the stacking of Ti₃C₂T_xsmall sheets resulted in ion transport obstruction.In this study,a new surface active site was introduced by doping g-C₃N₄to reduce the stacking of Ti₃C₂T_xlayers.The more nitrogen content of g-C₃N₄is a good choice of active site for metal ion adsorption.A simple acid treatment method is used to prepare more hydrophilic protated g-C₃N₄（H⁺）-Ti₃C₂T_xcomposite membrane which is easier to construct heterostructure with Ti₃C₂T_x.The electroadsorption properties of g-C₃N₄（H⁺）-Ti₃C₂T_xcomposite membrane in Na Cl,Mg Cl₂,and Al Cl₃solutions were tested by three electrode cyclic voltammetry（CV）.It was found that the Al Cl₃electrolyte has a unique REDOX peak and a higher specific capacitance of 192.8 F·g^-1.It is proved that the removal rate of the composite membrane in Al Cl₃solution is much higher than that of pure Ti₃C₂T_xfreestanding membrane.