Construction Of Ti₃C₂T_x MXene-based Faradaic Composite Electrode Materials For Hybrid Capacitive Deionization

With environmental pollution and population growth,the scarcity of freshwater resources has become an important global issue.Desalination technology has attracted great attention as a promising method to alleviate the shortage of freshwater resources.Capacitive deionization（CDI）is a novel and effective desalination technology,which is based on the formation of electric double layers（EDL）or faradaic reaction in electrochemical process to adsorb ions,and shows many advantages,such as low energy consumption,ease of operation and low cost.Electrode materials have always played an important role in determining the performance of CDI,and various carbon-based electrode materials have been used in CDI systems due to their large specific surface area and good chemical stability.However,conventional carbon-based CDI has some drawbacks such as low EDL-based desalination capacity(<20 mg g^-1),high energy consumption and severe anode corrosion,which limit its application in desalination.To solve these problems,hybrid capacitive deionization（HCDI）system was developed to replace one of conventional porous carbon electrode materials using Faradaic electrode material,showing better desalination performance.However,many current inorganic or organic faradaic electrode materials applied to HCDI suffer from insufficient electronic conductivity and low structural stability,which lead to some problems of Faradaic electrode material-based HCDI systems,including desalination capacity far below the theoretical capacity,poor cycling stability and low desalination rate,thus limiting their further commercial application.Ti₃C₂T_x MXene,as a novel two-dimensional pseudo-capacitive faradaic electrode material,has attracted more and more attention in the field of CDI due to its excellent electronic conductivity,large skeleton support structure,good hydrophilicity,and easy compounding with other Faradaic materials.However,there is a lack of in-depth exploration of MXene based composites in CDI,especially the intrinsic mechanism of MXene to enhance the desalination performance of Faradaic materials is not well defined,which limits the application of MXene in CDI.Therefore,in this paper,the accordion-like Ti₃C₂T_x MXene is prepared by HF etching and combined with various inorganic and organic Faradaic materials to construct a series of composites with unique three-dimensional network structures for HCDI applications,and the mechanism for the effect of MXene on the enhanced desalination performance of the composites was investigated.The main research contents are as follows:1.MoS₂,as a typical two-dimensional inorganic faradaic-type material,displays a unique S-Mo-S layered structure with large interlayer spacing,which could offer many ion storage sites.However,Mo S₂ exhibits a tendency of structural aggregation during electrochemical reactions,which leads to a significantly lower desalination capacity than the theoretical capacity and thus limits its desalination performance.Herin,the MoS₂/MXene heterostructure is constructed by in situ synthesis,and the aggregation of Mo S₂ is prevented by using two-dimensional MXene and two-dimensional Mo S₂ to build a three-dimensional"mutually supporting"structure,where Mo S₂ nanosheets can serve as"pillars"for the intrinsic structure of MXene,while MXene can provide structural reinforcement and electrical conductors for the structures.As expected,the HCDI system assembled from the Mo S₂/MXene exhibits a desalination capacity of23.98 mg g^-1 and a desalination rate of 4.6 mg g^-1 min^-1,showing a 44%increase in desalination capacity and a 31%increase in desalination rate compared to pure Mo S₂.2.NaTi₂（PO₄）₃（NTP）,as a typical inorganic faradaic material,has a higher electrochemical theoretical capacity than Mo S₂.However,the inherent low conductivity and high point-to-point contact resistance of NTP severely limit its kinetical and rate performance,which usually leads to poor desalination performance.Herin,Ti₃C₂T_x MXene was used as a Ti source for the synthesis of NTP,and part of MXene was converted into NTP by hydrothermal method.Na Ti₂（PO₄）₃/MXene（NTP/M）composites with three-dimensional network structure were successfully constructed.By utilizing the superior properties of both MXene and NTP,such as the high conductivity of MXene to reduce the charge transfer impedance and the abundant intrinsic ion channels of NTP to accelerate the ion transfer,the prepared NTP/M electrode materials exhibits excellent desalination performance,including a high desalination capacity of 64.4 mg g^-1and a desalination rate of 14.8 mg g^-1 min^-1,demonstrating a 76%increase in desalination capacity,and a 43%increase in desalination rate compared to pure NTP.3.Organic faradaic electrode materials represented by hexaazatrinaphthalene（HATN）have the advantages of eco-friendliness,low cost,tunable structure,electrochemical stability and high theoretical capacity of energy storage,however,their poor electrical conductivity limits their application in CDI.In this paper,three-dimensional HATN/MXene was fabricated by using ultrasound-assisted filtration.By introducing layered MXene as a highly conductive substrate and support,HATN,a one-dimensional linear organic molecule with aπ-conjugated system of multiple electroactive sites,can be randomly intertwined around the MXene layer to form a three-dimensional unique structure with more ion transport channels,thereby increasing the ion transport rate of the composite.The HATN/MXene shows a superior desalination performance,including a high desalination capacity of 57.5 mg g^-1 and a desalination rate of 13.2 mg g^-1 min^-1,exhibiting a 139%increase in desalination capacity and a 109%increase in desalination rate compared with pure HATN.4.The 2,6-diaminoanthraquinone（DAAQ）-triformylphloroglucinol（TFP）-covalent organic framework（COF）material（DAAQ-TFP-COF）,is a faradaic-type organic electrode material with redox active units,which has a higher electrochemical specific capacity than HATN.However,the inherent low conductivity of pure DAAQ-TFP-COF hinders the charge transfer within the framework,thus limiting its electrochemical performance and desalination performance.Herein,DAAQ-TFP-COF was grown in situ on layered MXene to obtain a three-dimensional unique network structure with abundant charge transfer pathways.The introduced two-dimensional layered MXene serves as a highly conductive substrate and large skeletal support to improve the conductivity,stability and specific surface area of COF.The COF/MXene structure shows an ultra-high desalination performance,including a desalination capacity of 78.8 mg g^-1 and a maximum desalination rate of 16.7 mg g^-1 min^-1,manifesting a 15%increase in desalination capacity and a 104%increase in desalination rate compared to pure COF.The study in this paper shows that Ti₃C₂T_x MXene provides skeletal support and charge transfer channels for faradaic electrode materials,and the constructed three-dimensional network structure provides more ion storage sites and higher charge transfer rates,thus enhancing the desalination capacity and desalination rates of faradaic electrode materials.The results of this paper provide a good guide to explore the application of MXene based faradaic composite electrode materials in CDI.