Design Of Capacitive Deionization Electrode Material And Its Application In Desalting/dechlorination Water Treatment

The current global supply and demand for freshwater resources is extremely unbalanced.In terms of demand,the global freshwater requirement continues to increase,but on the other hand,the world is facing a serious water shortage.The availability of abundant and cheap freshwater resources is therefore of great importance.The existence of abundant seawater resources on the earth,which has the advantage of being independent of climate,makes desalination an effective way to solve the limits of freshwater resources.Capacitive deionization technology（CDI）is gaining more and more attention for its simple structure,low energy consumption,regeneration and flexibility in system design.The ideal CDI electrode material should be suitable for fast ion transport,high conductivity,good chemical stability and low energy consumption.Currently,CDI electrodes are mainly made of carbon and Faraday materials,however,the development of CDI cathode materials is somewhat limited due to the lower energy density of carbon materials（bilayer capacitance）and the lower power density of Faraday materials（diffusion capacitance）.The design of cathode materials with both desalination capacity and rate remains a great challenge.On the other hand,there is less research on CDI anode materials,and the Bi³⁺/Bi⁰ conversion reaction for chloride ion removal suffers from imbalance in chloride and sodium ion storage and poor stability.Based on the above problems,three CDI electrode materials,VS₂/V₂CT_x,BMP-x and CF/Bi OCl,were prepared in this thesis as the cathode and anode of the capacitive deionization unit,respectively,and their sodium/chlorine removal performance and mechanism of action in the MCDI system were systematically investigated.The main results of our work are as follows:（1）MXenes have promising applications in capacitive deionization（CDI）owing to its excellent electrical conductivity,tunable layer structure and hydrophilic surface.Whereas,the electrochemical performance and water stability of MXene would be limited by a large number of terminations on its surface.Thus,we obtained VS₂/V₂CT_xnanocomposites by in-situ partial derivatization of MXene to enhance electrochemical performance and slow down its own oxidation,which was used as cathode for CDI desalination.The TOPSIS-Entropy Weigh model was used to achieve multi-objective optimization firstly,realizing a salt adsorption capacity of 166.83 mg·g^-1,a salt removal rate of 3.67 mg·g^-1·min^-1 and an energy consumption of only 0.256 kw·h·kg^-1-Na Cl.And it also showed a good stability over 60 CDI cycles.The fast salt removal rate and high desalination capacity of VS₂/V₂CT_x can be interpreted as the synergy between the pseudocapacitive and battery behaviors of VS₂/V₂CT_x hybrid material.In-situ electrochemical analysis of electrochemical quartz crystal microbalances（EQCM）showed that the desalination process is reversible and that the deionization mechanism consists of Faradic reaction（removal of Na⁺·4H₂O）and intercalation pseudo capacitance（removal of desolvated Na⁺）.In summary,VS₂/V₂CT_x,as a partially derived MXenes material,provides a new idea to address the problem of stability and electrochemical properties of MXene.（2）Based on the characteristics of MXene lamellar structure,Bi OCl nanostructures were grown on its surface/interlayer using a spatial confinement strategy,and BMP-x mixed electrodes with different MXene dosage were obtained.The fixed spacing alleviates the problems of capacity degradation and low stability of the material due to the volume expansion of Bi during the conversion process.The performance of the composite electrode is best when the MXene dosage is 10%.The percentage of diffusion-controlled Faraday reaction during charging/discharging decreases with increasing MXene mass fraction.After parameter optimisation the BMP-10 has a dichlorination capacity of up to 161.64 mg g^-1 and a dechlorination rate of 2.91 mg g^-1min^-1.The results of long-cycle experiments showed that the overall stability and reversibility of the conversion reaction of BMP-10 were good,with only minor side reactions at a voltage of about-1.2 V and at the reduction peak.The study of PDDA-modified MXene/Bi OCl hybrid electrodes for CDI dichlorination provides a new direction for the development of CDI anode materials.（3）To solve the problems of agglomeration and pore blockage caused by the presence of conductive agents and binders in conventional coated electrodes,Bi OCl nanoflowers were synthesized in situ using acid/alkali activated carbon felt（CF）as a substrate.The obtained CF/Bi OCl self-supported electrode facilitates the enhancement of the electrical conductivity of the material.The alkali-activated CF surface has more functional groups such as-OH and-CHO,which facilitates the pseudocapacitance reaction during the dichlorination of CDI.A linear fit of the power-law relationship shows that the conversion reaction of Bi³⁺/Bi⁰ and the pseudocapacitive reaction of chloride ions on the CF surface are the main modes of Cl^-removal,with a capacitive contribution of 48.7%and a diffusive contribution of 51.3%at a sweep rate of 0.05 V s^-1.The average volumetric dechlorination capacity under optimum operating conditions was up to 1.23 mg cm^-3,the average volumetric dechlorination rate was 0.02mg cm^-3min^-1 and the average energy consumption was only 0.45 kw h kg^-1-Na Cl.The multi-cycle charge/discharge long cycle dechlorination results demonstrate the good cycling stability of the alkali activated CF/Bi OCl electrode.These excellent properties make the self-supported CF/Bi OCl anode a potential binder-free CDI dechlorination material.