Preparation And Performance Evaluation Of Faradaic Pseudocapacitive Two-dimensional Intercalation Electrode Materials Based On MoS₂ And MXene For Capacitive Deionization Technology

Capacitive deionization technology（CDI）has attracted great attention in treating low-concentration saline water and brackish water for its environmental friendness and low-energy consumption.Electrode materials are the key component to the performance of CDI.The faradaic pseudocapacitive two-dimentional layered materials present significant advantages in the aspect of charge storage capacity,desalination rate,and applicable solution system,and have become the research focus in the CDI domain.However,the easy stacking property of two-dimentional layered faradaic pseudocapacitive materials caused by the strong interlayer interactions greatly reduces ion intercalation efficiency,affecting the desalination performance of CDI cell.Based on the two-dimensional layered pseudocapacitive material MoS₂ and MXene,four kinds of composite materials with different structural characteristics are prepared in this paper from the perspective of increasing the accessible ion intercalation sites,focusing on the effects of different material structures on improving the ion intercalation efficiency of electrode materials,and enhancing the ion removal capacity,ion transfer rate,ion selectivity,and structural stability of electrodes.Aiming at the problem of serious interlayer stacking and poor conductivity of MoS₂nanosheets,the 3D conductive network CNT-CS constructed by carbon nanotubes（CNT）and carbon sphere（CS）is used as the matrix for the well-dispersed uniform growth of MoS₂ on CS to prepare the composite MoS₂@CNT-CS.This conductive architecture endows MoS₂ with reduced layer number（3-6 layers）and enlarged ineterlayer space（0.67 nm）,promoting the electrical conducitivity and charge transfer rate of MoS₂.The desalination results indicate that the salt removal capacity of the hybrid CDI cell（HCDI）CNT-CS//MoS₂@CNT-CS(25.35 mg g^-1)is much higher than that of the cell CNT-CS//CNT-CS(18.36 mg g^-1)and CNT-CS//MoS₂(19.79 mg g^-1).And the cell CNT-CS//MoS₂@CNT-CS also displays the fastest desalination rate(3.9mg g^-1 min^-1)and excellent desalination capacity stability（30 cycles,98.8%）.To explore the ion selectivity of MoS₂@CNT-CS,the DFT results confirm that the lowest intercalation energy for Ca²⁺（-2.55 e V）in MoS₂ in the multi-salt solutions with four cations,indicating the most stable intercalation state of Ca²⁺in MoS₂@CNT-CS.And the cation adsorption capacity of MoS₂@CNT-CS electrode decreases as the order of Ca²⁺>Na⁺>Mg²⁺>K⁺,demonstrating the preferable pseudocapacitive deionization of MoS₂@CNT-CS to Ca²⁺over other cations.In order to prevent the interlayer self-stacking of MXene nanosheets,increase the accessible ion intercalation sites,and reduce the charge transfer resistance from the nonconductive binder,the binder-free layer-by-layer self-assembled MXene/CNT electrode is prepared by the electrophoretic deposition method with CNT as the interlayer spacer of MXene.Due to the addition of CNT,the interlayer space of MXene increases from 1.2 nm to 1.32 nm and the hydrophilicity of the deposited MXene/CNT is greatly improved（50°for contact angle）.Compared with the coating MXene-CNT electrode containing binder,the deposited MXene/CNT electrode presents the prominent specific capacitance(178 F g^-1),good specific capacitance retention rate（1500 times,90%）,and maximum Na⁺diffusion coefficient(2.97×10^-10 cm² s^-1).The desalination capacity of the CDI cell CNT//MXene/CNT(34.5 mg g^-1)is 1.3 and 1.8times that of the cell CNT//MXene(26.2 mg g^-1)and the cell CNT//MXene-CNT(18.7mg g^-1),respectively.Furthermore,the cell CNT//MXene/CNT achieves the fastest desalination rate(3 mg g^-1 min^-1)and lowest energy consumption(0.26 k Wh kg^-1-Na Cl).The 3D heterostructure MoS₂@MXene is synergistically constructed by MoS₂and MXene to enhance the conductivity of MoS₂,prevent the stacking of MoS₂ and MXene,and improve the structural stability of MXene.MXene can improve the dispersibility and conductivity of MoS₂ while MoS₂ as the protection layer not only prevents the self-stacking of MXene but also protects MXene from being oxidized,enhancing its structural stability.Compared with MoS₂ and MXene electrodes,the MoS₂@MXene electrode exhibits higher specific capacitance(171.4 F g^-1),lower series resistance（1.19Ω）,larger Na⁺diffusion coefficient(7.9×10^-8 cm² s^-1),and superior cyclic specific capacitance retention rate（1000 times,91.5%）.Due to the collaborative contribution of MoS₂ and MXene,the HCDI cell AC//MoS₂@MXene achieves significant desalination capacity(35.6 mg g^-1)and good cyclic desalination capacity stability（40 cycles,96%）.For the MoS₂@MXene electrode before and after desalination test,the morphology and structure analysis confirm that MoS₂ can shield the dissolved oxygen to prevent MXene from being oxidized and enhance its structural stability.To enhance the antioxidant capacity and ion selectivity of MXene,the composites PPy-Cl@MXene and PPy-DBS@MXene are prepared by in situ polymerizing Cl^-and DBS^--doped PPy with MXene.The self-stacking and antioxidant property of MXene can be greatly improved by wrapping with PPy layer.Furthermore,the abundant vacancy sites and the ion-exchange mechanism of PPy promote the ion selectivity of MXene.Compared with the MXene electrode,the two composite electrodes achieve enhanced ion storage capacity(241 and 235 F g^-1)and promoted pseudocapacitance contribution（89%and 89.4%）.The CDI cell PPy-Cl@MXene//PPy-DBS@MXene presents dominant desalination capacity(55 mg g^-1)and rapid desalination rate(15 mg g^-1 min^-1).The morphology and structure analysis of the two composite electrodes confirm that PPy layer protects the structural composition and surface charge properties of MXene from being affected by the dissolved oxygen after multiple desalination cycles.In the multi-cation mixed system,under the synergistic effect of ion intercalation of MXene and the ion-exchange process of PPy,the two composite electrodes PPy-Cl@MXene and PPy-DBS@MXene show a preferential adsorption to divalent cations(Ca²⁺and Mg²⁺).