Synthesis Of Modified MXene Electrode Materials And Its Application In Capacitive Deionization Desalination

Due to the rapid growth of the global population and the increasingly serious pollution of the water environment,the scarcity of fresh water has rapidly become a global problem,and desalination technology is considered to be the most practical way to solve the demand for fresh water.Among them,capacitive deionization（CDI）technology has attracted great attention due to its lower economic cost.Electrode materials are an important part of capacitive deionization systems,which mainly include carbonaceous materials and Faraday materials.Two-dimensional（2D）transition metal carbon/nitrogen compounds（MXene）,as a typical Faraday pseudocapacitive materials,has attracted extensive attention in recent years for its excellent conductivity and high capacity.However,MXene has some problems,such as easy agglomeration of lamellae,slow desalination rate and poor cycle stability.In order to solve the above problems,modification of MXene is particularly important.Atom/ion doping can expand the layer spacing of MXene,provide more ion channels and electrochemically active sites,effectively enhance its electrochemical performance,and improve its desalination capacity,rate,and cycle stability.MXene as a precursor can improve and maintain its excellent electrical conductivity and lamellar structure,realize the controllable preparation of various nanomaterials,enhance the specific surface area of derivative materials,and introduce more electrochemical active sites,thus increasing desalination capacity and efficiency.This paper focuses on the development of MXene modified electrodes and its capacitive deionization desalination.Typical transition metal carbides MXene（Ti₃C₂T_xand Mo₂Ti₂C₃T_x（T_xis usually-O,-F or-OH））are used as precursors.Sulfur doping and derivation methods are used to prepare sulfur-doped Ti₃C₂T_x（S-Ti₃C₂T_x）,Ti O₂/Ti S₂and Mo S₂/Ti S₂.The electrochemical properties of the above electrode materials and their application in capacitive deionization desalination are studied,and realized that MXene modified electrode materials have higher desalination capacity and desalination efficiency.The main research results as follows:（1）Sulfur-doped MXene（S-Ti₃C₂T_x）was prepared by hydrothermal method.By exploring the morphology of the material,S was successfully doped into the surface and between layers of MXene,maintaining the lamellar structure of MXene and expanding MXene.The interlayer spacing is conducive to the rapid transport and storage of sodium ions.The S-Ti₃C₂T_xelectrode is a pseudocapacitive material.It is obtained from the CV curve that S-Ti₃C₂T_xhas a higher oxidation peak and a lower reduction peak at about 0.1 V and-0.5 V,which is judged to be a Faraday storage mechanism.The value of the law relationship is close to 0.5,and it is further judged that the Na⁺storage reaction is mainly a Faraday intercalation reaction.The material was made into a CDI electrode,and the S-Ti₃C₂T_xelectrode was charged at a constant voltage with an applied voltage of 1.2 V.The maximum desalination capacity of the S-Ti₃C₂T_xelectrode was 152.3 mg·g^-1（Na Cl）,which was much higher than that of the MXene electrode.The effects of voltage,current density and initial concentration on the desalination capacity were investigated and a more efficient desalination environment was obtained by fitting the TOPSIS-entropy weight model as follows:the current density is30 m A·g^-1（that is,the current is 0.1-0.2 m A）,the chloride The initial sodium concentration was 12 m M and the applied voltage was 1.0 V.In this study,sulfur-doped modification of MXene and the expansion of layer spacing can effectively improve the desalination capacity,providing technical support and theoretical guidance for the application of MXene in capacitor deionization（2）In order to overcome the low stability problem of S-doped materials,the composite design of the materials is carried out.Using sulfur-doped S-Ti₃C₂T_xas the precursor,Ti O₂/Ti S₂nanosheets were synthesized by solid-phase method.SEM observed that Ti O₂/Ti S₂had a lamellar structure and long-coated rough particles between the lamellae,which proved that Ti O₂was successfully compounded on Ti S₂on the nanosheet,and make the material have a heterojunction structure.First,the stable tetragonal structure of Ti O₂effectively solves the problem of material stability.Compared with S-Ti₃C₂T_x,the specific surface area of??Ti O₂/Ti S₂nanosheets is significantly improved(32.21 m²·g^-1),which belongs to mesoporous materials and can provide channels for the rapid transport of sodium ions.The Ti O₂/Ti S₂nanosheets exhibit higher capacitance(49.51 F·g^-1)than that of S-Ti₃C₂T_x.According to the power-law relationship between current and scan rate close to 1.0,it is judged that the main mechanism of action is the Faraday surface redox reaction.The maximum desalination capacity of the Ti O₂/Ti S₂electrode was 36.45 mg·g^-1（Na Cl）,and the capacity retention rate was about 75%after 30 cycles,showing good reaction reversibility and cycle stability.The research realizes a new way of material compounding,and at the same time realizes the improvement of cycling stability of MXene-derived materials.（3）Using bimetallic Mxene（Mo₂Ti₂C₃）as the precursor,Mo S₂/Ti S₂was successfully prepared by solid-phase method.Electron microscope analysis showed that the surface of the electrode material was wrinkled and mesopores were dominant.Mo S₂/Ti S₂is a pseudocapacitive material obtained by electrochemical experiments,and the power-law relationship between current and scan rate is close to 1.0.Its adsorption mechanism mainly depends on the Faraday surface redox reaction.Further analysis according to EQCM shows that the reaction is reversible and is a material cyclic stability provides support.The maximum desalination capacity of Mo S₂/Ti S₂electrode is 69.4 mg·g^-1(current at 0.1-0.2 m A,15m L·min^-1solution flow rate and 1.2 V applied voltage environment),and it has an ultra-high desalination rate(3.68 mg·g^-1·min^-1)and good cycling stability.Studies have shown that the obtained Mo S₂/Ti S₂has excellent desalination content and cycle stability,which provides a choice for CDI desalination electrodes.In summary,this paper carried out research on Mxene modified electrode,and applied it in capacitive deionization desalination.Among them,sulfur-doped Ti₃C₂T_x（S-Ti₃C₂T_x）has ultra-high desalination content,Ti O₂/Ti S₂nanosheets have better cycling stability,and Mo S₂/Ti S₂performs well in desalting capacity and cycling stability.High desalination capacity and desalination efficiency have potential application prospects in CDI desalination technology.