Study On The Hydro-thermal Synthesis And Capacitive Deionization Performance Of Sodium Ion Insertion-deinsertion Electrode

In order to tackle the urgent problem of global freshwater shortage,the development of economically attractive desalination technologies has been the subject of recent attention.Over the years,a number of desalination methods have been developed among which distillation,reverse osmosis,and electrodialysis are the most commonly known and widespread technologies.Compared to these established desalination technologies,a promising technology called Capacitive Deionization(CDI)has emerged as a robust,energy efficient,and cost effective technology for desalination of water with a low or moderate salt content.CDI has two kinds of ion storage mechanisms:capacitance and Faradaic reaction(pseudocapacitance).The former is stored in the interior of the electrode via an electric double layer and porous carbon is used as the electrode material.The latter stores ions by a fast Faradaic reaction that takes place at the surface of the electrode of transition metal oxide.The desalination performance of CDI is closely related to the electrode material and the ion storage mechanism.However the desalination capacity for CDI is still relatively low as compared to commercial reverse osmosis and this technique is not suitable for the deionization of high concentration saline water.Exploring new materials is one way to increase the desalination capacity of CDI.In this study,an asymmetrical electrosorption desalination(A-CDI)device consisting of an oxide inlay electrode and activated carbon was used.The sodium ion electrode is used as the cathode,and the cation(sodium ion)is captured by the Faradaic reaction;the anion(chlorine,etc)is captured by electrosorption using the porous carbon material as the anode.The system introduces Faradaic reaction during desalination in the cathode,which is expected to increase the desalination capacity and charge efficiency.The main research contents of this paper are as follows:1.The rod-like mesoporous material Na0.7MnO2(NMO)with a diameter of 200 nm and a length of 4?m was prepared by hydrothermal method.Electrochemical results show that the specific capacitance of NMO is 120.09 F/g when the scan rate is 1 mV/s in 1 M Na2SO4 solution,and the highest specific capacitance can reach 49.22 F/g at the current density of 0.5 A/g.NMO was used as the negative electrode and activated carbon was used as the positive electrode to form an asymmetrical capacitive deionization system(A-CDI).Desalination performance was tested in NaCl,NaN03 and Na2S04 solutions with an initial conductivity of 500 ?S/cm.The electrosorption capacity attained 0.183,0.178 and 0.124 mmol/g,respectively.Since the valence of SO42-was 2,the actual capacity of A-CDI in Na2SO4 was 0.248 mmol/g after normalized equivalence capacity.The experimental results were in agreement with the hydration radius of anions(SO42-<Cl-<NO3-),showing preferential electrosorption of anions by AC/NMO is related to ion hydration radius and valence.After 5 cycles of adsorption-desorption,there was no significant decrease in removal capacity,indicating the superior regeneration.2.Na4Ti9O20(NTO)nanotubes were prepared by hydrothermal method.It was found that the morphology of NTO gradually changed from sheet to nanotube with the increase of hydrothermal temperature and reaction time:when the temperature was 200 ? and the time was 4 h,the product was a pure phase NTO nanotube that is about 10 nm in diameter and 300?500 nm in length.Moreover,the BET specific surface area of NTO can reach 142.432 m2/g.At the same time,the pore volume is 0.601 cm3/g and the mean pore size is 2.789 nm,respectively.Electrochemical studies have shown that NTO has a specific capacitance of 120.45 F/g at a scan rate of 1 mV/s and an initial discharge capacity of 48.56 F/g at a current density of 0.5 A/g.The results of CDI study showed that the electrosorption capacity of AC/NTO was 23.35 mg/g and the charge efficiency reached 84.45%.3.The Graphene@Na4Ti9O20(rGO@NTO)composite was synthesized by one-step hydrothermal method.The results show that rGO can adhere well to NTO and conmect the NTO nanotubes together.Through EIS analysis,it can be seen that the resistance of rGO@NTO is significantly reduced,which is more conducive to the occurrence of electrosorption.Electrochemical studies have shown that the specific capacitance of rGO@NTO at a scan rate of 1 mV/s can attain 125.2 F/g and it was still maintained 33.94 F/g when the scan rate reached 100 mV/s,indicating that the capacitance retention rate was 20.48%higher than that of pure NTO.Besides,the initial discharge capacitance was 55.60 F/g at a current density of 0.8 A/g.The CDI results showed that the maximum electrosorption capacity of AC/rGO@NTO was 41.8 mg/g and the charge efficiency was as high as 99.24%.Futhermore,electrosorption capacity and charge efficiency are not only higher than AC/NTO,but also much higher than other related electrode materials(carbon materials,graphene,metal oxides,MOFs).In addition,the dynamic adsorption process of rGO@NTO accords with the Lagergren pseudo-first-order kinetics adsorption equation.And after repeated tests,the electrode showed good regeneration performance.