Preparation Of Hydrotalcite-based Hybrid Electrode Materials And Its Performance In Capacitive Deionization

Capacitive deionization（CDI）is an emerging and evolving water treatment technology with a wide range of application prospects in recent years.It has the advantages of economic efficiency,environmental friendliness and simple operation.CDI has been widely used for the removal of various ions from water（such as nitrogen,fluoride and phosphorus）.Layered double hydroxides（LDHs）are a class of anionic layered compounds,also known as hydrotalcites,which have been used as CDI electrodes to study the removal of anions from water due to their large specific surface area,good chemical stability and excellent ion exchange capacity.Studies have shown that LDHs are often combined with carbon materials to improve their CDI properties.Therefore,in this paper,a series of hydrotalcite-activated carbon fiber（ACF）-based hybrid electrode materials were prepared and their performance as CDI electrodes for the removal of phosphate ions from water was investigated.The specific studies are as follows:（1）Hydroxide-activated carbon fiber-based hybrid electrode materials（Ni Fe-LDH/ACF-1,Ni Fe-LDH/ACF-2）were synthesized by hydrothermal method.It was also characterized by a series of tests using X-ray powder diffraction（XRD）,scanning electron microscopy（SEM）,cyclic voltammetry（CV）,electrochemical impedance test（EIS）and N₂ adsorption-desorption isotherms.The results of CDI experiments showed that the phosphate ion adsorption performance of Ni Fe-LDH/ACF-2 hybrid electrode was better than that of Ni Fe-LDH/ACF-1,which may be related to the loading of Ni Fe-LDH and its higher specific surface area.The phosphorus adsorption capacity reached 33.48 mg/g at an applied voltage of 2.4 V.The fitted curve study by adsorption kinetic model and adsorption isotherm model showed that the applied voltage was positively correlated with the phosphorus adsorption capacity and it was a monolayer adsorption occurred on the surface of CDI electrode.Meanwhile,the prepared hybrid electrode has good reusability and stability.（2）La-doped Ni Fe-LDH/ACF hybrid electrodes（NFL_x/ACF）with different M³⁺/M²⁺molar ratios were successfully synthesized by hydrothermal method.The hybrid electrodes were characterized by XRD,SEM,CV,EIS,and X-ray photoelectron spectroscopy（XPS）.The experimental results of CDI showed that the phosphorus adsorption performance of the CDI electrode was enhanced by La-doped,which was attributed to the enhanced pseudocapacitive behavior of the CDI electrode.the phosphorus adsorption capacity of NFL_1/3/ACF reached31.82 mg/g.The results show that the adsorption reaction of phosphate ions on the hybrid electrode conformed to the pseudo-second-order adsorption kinetic model,and the phosphate ions undergo monomolecular layer adsorption on the surface of the NFL_1/3/ACF hybrid electrode.After ten cycles of experiments,the CDI electrode NFL_1/3/ACF could still adsorb phosphate ions effectively.It indicates that the hybrid electrode has good reusability.（3）A new hybrid electrode Ni Fe-LDH/ACF-Fc_x was formed by adding ferrocene（Fc）as an electron shuttle to the surface of the hybrid electrode Ni Fe-LDH/ACF.By adding ferrocene to the electrode material,the phosphorus adsorption capacity in the CDI process reached a great improvement.This is due to the fact that the addition of ferrocene accelerates the reversible redox reaction occurring at the electrode/electrolyte interface,which increases the electron transfer rate and ion mobility,thus improving the removal of phosphate.The phosphorus adsorption capacity of Ni Fe-LDH/ACF-Fc₅ reached 29.12 mg/g.